Image SEO: Alt Text, File Names and Compression Tips

Image SEO is frequently neglected, yet images represent a significant organic traffic opportunity through Google Image Search and can directly impact your page ranking through their effect on page speed and Core Web Vitals. Getting image SEO right requires attention to file format, size, alt text and structured data.

Alt text: the most important image SEO element

Alt text (alternative text) describes an image to search engines and screen readers. Write descriptive alt text that accurately describes what is in the image. Include the page's primary keyword where it occurs naturally in the image description. Do not keyword-stuff alt text. Decorative images that add no informational value should have empty alt attributes (alt="") so screen readers skip them. Every informational image should have unique, descriptive alt text.

File format and compression

Use WebP as your primary image format: it delivers 25-35% smaller file sizes than JPEG at comparable visual quality, with full browser support. Use JPEG for complex photographs where WebP is unavailable. Use PNG for graphics with transparency. Use SVG for logos and icons. Compress all images before upload: Squoosh (free, browser-based) or ShortPixel/Imagify for WordPress (automatic on upload). Target file sizes under 150KB for standard images, under 50KB for thumbnails.

File naming and dimensions

Name image files descriptively with hyphens between words: seo-audit-checklist.webp, not IMG_2947.jpg. Include the primary keyword where relevant and accurate. Always serve images at the dimensions they will be displayed - do not serve a 2000px image in a 400px container and rely on CSS to scale it down. Use responsive images (srcset attribute) to serve appropriately sized images based on the user's screen resolution.

Lazy loading and CLS prevention

Add loading="lazy" to all images below the fold - this defers loading until the image is about to enter the viewport, improving initial page load time and LCP. Use loading="eager" on the hero or first image above the fold (the likely LCP element). Always specify explicit width and height attributes on every img tag - this allows browsers to reserve space before the image loads, preventing Cumulative Layout Shift (CLS).

Image structured data

For images that are the primary subject of a page or article, add ImageObject schema to make them eligible for Google Image Search rich results. For product images, add Product schema with image properties. For recipe images, add Recipe schema. These schema types increase the likelihood of images appearing in Google's visual search features and rich result formats.