Image SEO: Alt Text, File Names and Compression

Images are the most neglected asset in on-page SEO — uploaded as IMG_4471.jpg, alt text blank, two megabytes heavy, and then everyone wonders why the page crawls and image search sends nothing. Image SEO is three small disciplines — naming, describing, compressing — that pay in four currencies: accessibility, image-search traffic, page speed and context signals. Fifteen minutes of habit, learned once. Here's the whole practice.

Why images matter more than their effort suggests

Four separate payoffs stack. Accessibility: alt text is how screen readers convey images — a legal and ethical baseline before it's an SEO one. Image search: Google Images is a top-tier traffic source in visual niches (products, recipes, design, how-tos), ranked substantially on the signals below. Speed: images are most pages' heaviest bytes, and page experience metrics price that weight. Context: filenames, alt text and captions feed the page's relevance picture — and, as the image link building guide showed, well-attributed images become citable assets that earn links on their own.

Discipline 1: file names

Name the file what it shows, hyphenated, before upload: anchor-text-distribution-chart.png, not Screenshot 2020-02-12.png. It's a modest signal to image search and a large mercy to your own media library. Batch-renaming after upload is miserable; the habit at save time is free — same economics as URLs, smaller stakes.

Discipline 2: alt text

The alt attribute describes the image for whoever (or whatever) can't see it. The craft in four rules:

• Describe what's actually shown, specifically. "Line chart showing referring domains growing from 40 to 220 over six months" beats "chart" and demolishes "".
• Write for the listener, not the algorithm. The screen-reader test — would this sentence usefully replace the image? — produces correct alt text automatically, keywords included when honest.
• Keywords only where true. If the image genuinely depicts the topic, the phrase belongs; "backlink audit checklist template" on a stock photo of a laptop is stuffing, and image spam reads as page spam.
• Decorative images get empty alt (alt="") — deliberately, so screen readers skip them. Empty-by-decision differs from empty-by-neglect.

One special case worth repeating from the link building curriculum: when an image is a link, its alt text serves as the anchor text — write it as such.

Discipline 3: weight and delivery

1. Right format: WebP as the modern default (photos and graphics both), JPEG the safe fallback, PNG only where transparency or pixel-exact UI matters, SVG for logos and icons.
2. Right dimensions: resize to the largest size actually displayed — shipping 4000px camera output into a 800px column is the classic megabyte leak.
3. Compression, automated: lossy at sane quality (~75–85) is visually free and hugely lighter; make it a build step or CMS plugin, not a willpower exercise.
4. Lazy-load below the fold (loading="lazy" is a one-attribute win) and set width/height so the page doesn't jump as images arrive — layout shift is a measured experience metric.

The supporting cast

Captions are read far more than body text — use them where an image needs interpretation (and they're where image credits live, feeding the reclamation pipeline when others reuse your visuals). Structured placement matters: an image near the text it illustrates inherits that context. Image sitemaps help large visual catalogues get discovered; blogs at this site's scale don't need them. Original images beat stock everywhere it counts — uniqueness ranks in image search, and originals are the only ones that earn citation links back.

Frequently asked questions

How long should alt text be?

A sentence-ish — long enough to genuinely describe, short enough to listen to; ~125 characters is a fair ceiling from screen-reader convention. Complex charts get a short alt plus a fuller description in surrounding text.

Does image SEO really move rankings for normal pages?

Directly, modestly: speed and experience effects are real, context effects small. The step-change is in visual niches via image search itself, and for everyone via the asset-earning route — an original chart that earns citations out-ranks any alt-text tweak by orders of magnitude.

Stock photos: rename and alt-text them anyway?

Yes for accessibility, honestly for description — but know the ceiling: a stock photo carries no uniqueness signal and earns no links. This series' own merit principle applies to pixels too. Next up, the highest-leverage discipline of the silo: internal linking — and the external half stays where it always is, in our website database and inbox.