SEO Glossary: Image & Media Optimization

Ninth entry in the running SEO glossary and the first of three posts covering the terms originally bundled under a single “24 essential terms” post. This one groups the four terms that are genuinely about the same subject: images and media. This installment covers images specifically; video and audio optimization terms will follow in a later glossary post.

Image Alt Text

Alt text is the HTML attribute that provides a text description of an image, serving two purposes at once: it’s read aloud by screen readers for visually impaired users, and it gives search engines textual context about what an image shows, which matters for both general page understanding and image search rankings specifically.

There’s no strict, enforced character limit for alt text, and claims of a hard cutoff are technically a myth: the commonly repeated figure of roughly 100-125 characters traces back to how an older screen reader, JAWS 6.0, chunked long alt text into separate readable segments rather than any actual limit imposed by current assistive technology or by search engines. That said, the underlying practical guidance still holds up: keeping alt text to roughly one or two sentences, generally under about 125 characters, tends to produce more usable descriptions for both screen reader users and search engines than long, padded strings do, even without a hard technical requirement forcing that length. Alt text should describe what the image actually shows, without redundant phrasing like “image of,” and purely decorative images should carry an empty alt attribute (alt="") so screen readers skip over them. Complex images that need more explanation than a short alt attribute can reasonably hold, such as a data chart, are better served by a visible caption or an aria-describedby reference to a longer description elsewhere on the page.

Image Compression

Image compression reduces file size through either lossy compression, which discards some image data to shrink the file further, or lossless compression, which preserves all original data at a smaller but less dramatic size reduction. Images still commonly account for the largest single resource category on a typical page, roughly a third to 40% of total byte weight according to recent HTTP Archive Web Almanac data, which makes compression one of the highest-leverage levers for improving load speed even though that share has shrunk somewhat as JavaScript and font payloads have grown. Well-executed compression can commonly cut file sizes by somewhere in the range of 50-80% with no visually noticeable quality loss, which matters directly for Largest Contentful Paint, the Core Web Vitals metric with a target of 2.5 seconds or less, since a page’s largest visible element is very often an image.

Format choice affects both the compression ceiling and quality tradeoff: JPEG and lossy WebP suit photographs well, PNG and lossless WebP suit graphics needing perfect fidelity or transparency, and modern formats like WebP and AVIF generally outperform older formats like JPEG and PNG on file size at comparable visual quality. SVG remains the right choice specifically for vector graphics like logos and icons rather than photographic content.

Image SEO

Image SEO is the umbrella term for the full set of practices that make images discoverable and performant within search: descriptive file naming instead of generic camera-generated filenames, proper alt text implementation, appropriate format selection for the content type, compression and general performance optimization, responsive image delivery using srcset and sizes attributes so different devices load appropriately sized versions, lazy loading for images below the initial viewport, image sitemaps for large or complex image libraries, and structured data markup where relevant, such as for product images. None of these individually determines image search visibility on its own; they function together, and neglecting any one of them, particularly alt text or basic compression, tends to undercut the benefit of getting the others right.

A product photo file named “IMG_4821.jpg” with no alt text, served at full camera resolution with no srcset, is invisible to image search and actively slows the page. Renaming it to “blue-ceramic-planter-8-inch.jpg,” compressing it, and adding descriptive alt text addresses three of the eight practices in one pass.

Image Sitemap

An image sitemap is a specialized XML sitemap, or an extension of a standard sitemap, that lists the images on a site so search engines can discover images that might otherwise be missed, particularly those loaded dynamically through JavaScript or buried deep in a site’s structure. Google’s currently supported image sitemap markup is deliberately minimal: the <image:image> and <image:loc> tags are what Google actually reads. Older guidance describing an image sitemap as a place to add caption, title, geo-location, or licensing metadata is out of date; Google deprecated those supplementary tags in 2022, and while leaving them in an existing sitemap does no harm, adding them now serves no indexing purpose. Google’s practical limits for a single sitemap file are 50,000 URLs or 50MB uncompressed, reduced to a 10MB limit if the file is compressed, and sites exceeding those limits need to split entries across multiple sitemap files referenced from a sitemap index. An image sitemap should only reference images that appear on pages that are themselves indexable, since listing images tied to noindexed or blocked pages provides little practical benefit.

Before assuming a slow LCP score is a compression problem, confirm the largest visible element actually is an image using PageSpeed Insights’ element-level breakdown. The fix only helps if images are actually the bottleneck.

Quick reference

Term	One-line definition
Image Alt Text	Text description of an image, primarily for accessibility
Image Compression	Reducing image file size to speed up page load
Image SEO	The broader practice of making images discoverable and fast-loading
Image Sitemap	A specialized sitemap listing a site's images for discovery

Sources cited: Google Search Central: image best practices, Google Search Central: image sitemaps, web.dev: image optimization and Core Web Vitals