Image Optimisation for SEO
Image optimization affects page speed (a ranking factor), appears in Google Images (a traffic source), and provides context for screen readers. Well-optimized images are smaller, load faster, and are more discoverable.
Alt Text: What It Is and Why It Matters
Alt text (alternative text) is a short description of an image displayed when the image fails to load and read aloud by screen readers. It serves both accessibility and SEO purposes.
Write alt text that genuinely describes the image for someone who can't see it. If your image shows a person making sourdough, good alt text is "woman pouring water into sourdough dough in a glass bowl." Bad alt text is "sourdough" or "image123" or "keyword keyword keyword."
Google uses alt text to understand what images depict, which helps it rank images in Google Images Search. More importantly, clear alt text improves accessibility for screen reader users, which is both an ethical obligation and increasingly a legal requirement.
Include your target keyword in alt text when it genuinely fits the image description. "How to make sourdough bread: woman mixing dough in a bowl" is descriptive alt text that naturally includes your keyword. "Best keyword research tools keyword research software" is forced and unhelpful.
File Names Matter
Use descriptive file names instead of generic ones. sourdough-starter-preparation.jpg is better than IMG_3847.jpg or image1.jpg. Google uses file names as a weak ranking signal for image search, and descriptive names are more informative to crawlers.
Separate words with hyphens, not underscores or spaces. sourdough-bread-recipe.jpg works. sourdough_bread_recipe.jpg or "sourdough bread recipe.jpg" creates problems.
Compression and File Size
Uncompressed images are one of the biggest page speed killers. A single unoptimized 5MB image can slow your page load time by seconds, which damages your ranking (Core Web Vitals) and user experience.
Compress all images before uploading. Most images can lose 60-80% of their file size with imperceptible quality loss. Tools like ImageOptim (Mac), Optimizilla (online), or Squoosh (Google's free tool) make this simple.
The goal is the smallest file size that maintains acceptable visual quality. For photos, JPEG compression is usually fine. For graphics, PNG is better. Lossless compression is ideal; lossy compression (which loses some data) is acceptable if quality is maintained.
Next-Gen Image Formats
WebP and AVIF are modern image formats that significantly outperform JPEG and PNG. A WebP file can be 25-35% smaller than a comparable JPEG with no quality loss.
You can serve images in multiple formats using the HTML picture element or srcset attribute. Modern browsers get WebP or AVIF; older browsers fall back to JPEG or PNG. This requires minimal effort and provides real performance gains.
Most CDNs and image optimization services (Cloudflare, Imgix, Smush) can automatically convert and serve images in optimal formats. If you're using WordPress, plugins like Smush or ShortPixel handle this automatically.
Lazy Loading
Lazy loading defers the loading of off-screen images until they're about to be visible. Instead of loading all images when the page loads, lazy loading only loads images as users scroll toward them.
This improves initial page load time significantly, especially on image-heavy pages. The HTML attribute is simple: <img loading="lazy" src="image.jpg" alt="description">
Browser support is excellent now (all modern browsers support lazy loading). Use it on all images below the fold (images not visible when the page first loads).
Image Sitemaps and Google Images
Images can be a significant traffic source if optimized for Google Images. Create an image sitemap that includes all images on your site. Google will crawl it and index your images.
In the image sitemap, include image URL, page URL (the page the image appears on), title, and caption. This metadata helps Google understand the context and relevance of your images.
Most SEO plugins (Yoast, Rank Math, All in One SEO) automatically generate image sitemaps. If you're not using a plugin, you can use a tool like Screaming Frog to generate one.
Responsive Images
Serve different image sizes for different devices. A desktop browser doesn't need the high-resolution 4000px wide image that a mobile phone can't even display. Use srcset and sizes attributes to serve appropriately sized images.
This saves bandwidth on mobile devices (improving performance) and ensures images display crisply on high-resolution screens (improving user experience). Most modern CMSs and image CDNs handle this automatically.
Image SEO Checklist
- Use descriptive file names (sourdough-starter.jpg, not IMG_123.jpg)
- Write descriptive alt text (genuine description, not keyword stuffing)
- Compress images before uploading (60-80% reduction is typical)
- Use next-gen formats like WebP when possible
- Implement lazy loading on below-the-fold images
- Serve responsive images (different sizes for different devices)
- Include images in your sitemap or use an image sitemap
- Use relevant images — avoid generic stock photos that don't add value
Common Image Mistakes
Keyword-Stuffed Alt Text
"Best keyword research tools, affordable keyword research software, free keyword research tools, keyword research tips" is terrible alt text. It's unreadable to screen reader users and obvious spam to Google. Write genuine descriptions.
Empty Alt Text
Some images don't need alt text (purely decorative images). But many images that are left with empty alt text should be described. Check your images — if it conveys information, it needs alt text.
Massive, Uncompressed Files
A single 8MB image can tank your page speed. Most images should be under 1MB, many can be under 100KB. Compress ruthlessly.
No Responsive Images
Serving a 4000px desktop image to a mobile phone wastes bandwidth and slows load time. Use responsive image techniques to serve appropriately sized images.