A single unoptimised image can slow down your entire webpage. And a slow page does not just frustrate visitors, it pushes your rankings down.
Images account for more than 50% of the average webpage’s total weight, according to HTTP Archive data. That means how you handle images has a direct impact on how fast your site loads, how Google evaluates your pages, and whether potential customers stay or leave.
This guide covers everything you need to know about image optimisation for SEO — from choosing the right file format to writing alt text that actually helps your rankings.
What This Guide Covers
- Why image optimisation directly affects your search rankings and page speed
- Which image format — JPG, PNG, WebP, or AVIF — is best for SEO in 2025
- How to compress images without sacrificing quality
- How to write alt text that helps both Google and real users
- What lazy loading is, when to use it, and when to avoid it
- A ready-to-use image SEO checklist for WordPress sites
Why Image Optimisation Matters for SEO
Google’s Core Web Vitals include a metric called Largest Contentful Paint, or LCP. It measures how long it takes for the biggest visible element on a page to load. In most cases, that element is an image.
A poor LCP score signals to Google that your page is slow. That affects your ranking, particularly on mobile, where Google applies mobile-first indexing to evaluate your site.
Page speed also has a direct impact on user behaviour. According to Google’s own research, the probability of a visitor bouncing increases by 32% when page load time goes from one second to three seconds. For small business websites where every visitor counts, that is a number worth taking seriously.
The main purpose of image optimisation is straightforward: reduce file size and load time without compromising visual quality, so your pages load fast and rank well.
Which Image Format Is Best for SEO?
Is JPG or PNG Better for SEO?
Neither JPG nor PNG is the best choice for most websites in 2025. Both have been largely superseded by WebP, a format developed by Google that delivers smaller file sizes at comparable visual quality.
Here is how each format works and when to use it:
JPG – best for photographs and complex images with many colours. It uses lossy compression, which means some image data is permanently removed to reduce file size. JPG does not support transparent backgrounds.
PNG – best for logos, icons, and screenshots where sharpness matters. It uses lossless compression, so no image data is discarded. PNG supports transparency, which makes it the right choice when your image needs a clear background.
WebP – the practical default for most website images in 2025. It supports both lossy and lossless compression, handles transparency, and produces files that are typically 25–34% smaller than JPG at equivalent quality, according to Google’s own benchmarks. All modern browsers support it.
AVIF – the next generation after WebP, with even better compression ratios. Browser support has reached approximately 85% in 2025 and is still growing. It is worth monitoring, but WebP remains the safer default for now.
For most WordPress websites, WebP is the right choice. Plugins like ShortPixel and Imagify convert your uploaded images to WebP automatically, so you do not need to do it manually for every file.
Image Compression: How Small Is Small Enough?
What Is the Best Image Size for SEO?
There is no single correct file size, it depends on where the image appears. A hero image that spans the full width of a desktop screen has different requirements than a small thumbnail in a blog post sidebar.
These are reasonable targets to work towards:
- Hero images: under 200KB
- Blog post inline images: under 100KB
- Thumbnails: under 30KB
Is 72 or 300 DPI Better for Web?
72 DPI. Screen resolution does not benefit from higher DPI values, 300 DPI is for print. Uploading a 300 DPI image to your website adds unnecessary file size with no visual benefit to the viewer.
Lossy vs Lossless Compression
Lossy compression reduces file size by permanently removing some image data. Lossless compression reduces size without discarding any data. For most website images, lossy compression at 75–85% quality is the right balance, the quality difference is invisible to the human eye, but the file size reduction is significant.
Tools worth using: Squoosh (free, browser-based), TinyPNG (free tier available), and ShortPixel (WordPress plugin that compresses images automatically on upload).
Alt Text: How to Write It the Right Way
Why Is Alt Text Important for SEO?
Alt text serves two purposes. First, it tells Google what an image contains — since search engines cannot “see” images the way humans do. Second, it is read aloud by screen readers for visually impaired users, which means it also affects your site’s accessibility and WCAG compliance.
Getting alt text right is one of the highest-return, lowest-effort improvements you can make to your website.
The Formula
[What the image shows] + [context if relevant] + [keyword if it fits naturally]
Here is what that looks like in practice:
| Image | Bad Alt Text | Good Alt Text |
| A plumber fixing a kitchen pipe | image1 | plumber repairing kitchen sink pipe |
| A bar graph showing traffic growth | graph | SEO traffic growth chart showing 3x increase over 6 months |
| Your business logo | logo | Rhasko Digital logo |
Never stuff keywords into alt text. If the keyword does not fit naturally into a description of what the image actually shows, leave it out.
When to Leave Alt Text Empty
Decorative images such as backgrounds, dividers, and purely aesthetic graphics should have an empty alt attribute (alt=””). This tells screen readers to skip the image entirely, which is the correct behaviour for images that carry no informational value.
Image File Names: The Quick Win Most SMBs Miss
When you upload an image straight from your phone or camera, it usually arrives with a name like IMG_4892.jpg or DSC_0047.png. Google reads file names as a signal about image content. A generic file name tells it nothing.
Rename your images before uploading them. Use lowercase letters, separate words with hyphens, and include a relevant keyword where it fits naturally.
plumber-sydney-kitchen-pipe-repair.jpg tells Google far more than IMG_4892.jpg ever could.
Image Dimensions and Responsive Images
Uploading a 3,000-pixel-wide image to fill a 600-pixel column forces the browser to download far more data than it needs. The image displays at the right size visually, but the excess file weight still loads in the background.
The fix is to serve appropriately sized images for different screen sizes using the srcset attribute in HTML. You do not need to write this code manually because WordPress generates multiple image sizes automatically when you upload a file.
Where WordPress falls short is when you upload images that are already undersized, or when your theme is not configured to use the correct image sizes for each layout element. Check your theme documentation to confirm which image dimensions it expects for each slot like header, featured image, thumbnail, and so on.
Lazy Loading: What It Is and How It Affects SEO
What Is Lazy Loading?
Lazy loading is a technique that delays the loading of images until the user scrolls close to them. Instead of loading every image on a page the moment someone visits it, the browser only loads what is immdiately visible, then loads the rest as needed.
This reduces initial page load time, which improves your Core Web Vitals score and the experience for users on slower connections.
Lazy Loading vs Eager Loading
Eager loading is the default browser behaviour where everything loads at once. Lazy loading is the alternative. For most images below the fold (below what is visible without scrolling), lazy loading is the better choice. For images above the fold, eager loading is correct.
Native lazy loading is now supported across all major browsers. Chrome, Firefox, and Edge using a simple HTML attribute: loading=”lazy”.
WordPress has had lazy loading enabled by default since version 5.5, so most WordPress sites are already benefiting from it without any additional setup.
When Not to Use Lazy Loading
Do not apply lazy loading to your hero image or any image that is visible immediately when the page loads. Lazy loading these images delays their appearance and directly harms your LCP score, the opposite of what you want.
Image Sitemaps and Indexability
An image sitemap tells Google which images exist on your site and where to find them. It does not guarantee indexing, but it does make discovery easier, partcularly for images that are loaded via JavaScript or embedded in ways Google’s crawler might otherwise miss.
For most WordPress sites using Yoast SEO or Rank Math, image sitemap data is generated automatically as part of your main sitemap. You do not need to create a separate one.
To check which of your images Google has actually indexed, open Google Search Console and navigate to the URL Inspection tool. Enter a page URL and check the indexed content details.
Schema Markup for Images (For AI Search Visibility)
Schema markup, specifically the ImageObject type will gives search engines and AI systems structured information about your images. This increases the likelihood of your images appearing in rich results and being cited in AI-generated answers and Google AI Overviews.
Here is a basic example:
json
{
“@context”: “https://schema.org”,
“@type”: “ImageObject”,
“url”: “https://yourdomain.com.au/images/seo-audit-checklist.webp”,
“name”: “SEO audit checklist for small business websites”,
“description”: “A visual checklist showing the key steps in an SEO audit for small business websites.”
}
Yoast SEO and Rank Math both add basic image schema automatically. For more control, you can use a dedicated schema plugin or add the markup manually to your page template.
Image SEO Checklist for WordPress Sites
Use this before publishing any new page or post:
- Image saved in WebP format
- File size within target (hero under 200KB, inline under 100KB, thumbnails under 30KB)
- File name uses descriptive keywords with hyphens, no generic names
- Alt text written using the description-plus-context formula
- Decorative images have empty alt attributes
- Image dimensions match the layout slot, no oversized uploads
- Lazy loading active on all below-the-fold images
- Hero image set to eager loading (not lazy)
- Image sitemap confirmed active in Yoast SEO or Rank Math
- ImageObject schema added where relevant
Conclusion
Image optimisation is one of the few areas of SEO where the effort-to-impact ratio works strongly in your favour. The changes are mostly one-time — rename your files correctly, compress before uploading, write proper alt text — and the performance benefits compound over time as Google re-crawls your pages.
Start with an audit of your existing images. Use Google Search Console to check your Core Web Vitals score, identify pages with poor LCP performance, and work through those pages first.
If you would like help optimising your WordPress site for search, images included, get in touch with Rhasko Digital.