Image SEO: Alt Text, Formats, and Compression Done Right
Image SEO is a three-part kit: write alt text that describes the image in one sentence (working the keyword in naturally, never stuffing); prefer WebP or AVIF for the format; and compress content images to ≤ 150KB while lazy-loading anything below the fold. Get these three right and you earn Google Images traffic, an accessibility boost, and better Core Web Vitals — all at once.
Why image SEO is worth the effort
- Google Images is an underrated traffic door. For recipes, tutorials, and product content, image search often accounts for 10–20% of organic traffic.
- Your image is usually the LCP element. An article’s hero image is almost always the “largest contentful element” — if it isn’t optimized, your CWV Largest Contentful Paint won’t be good (the threshold is 2.5 seconds).
- AI engines read alt too. When multimodal models and AI search parse a page, alt text is their first-hand source for what an image means.
Move one: alt text
Alt (alternative text) exists first for accessibility — a screen reader reads it aloud to blind users. Its SEO value is that the same sentence tells search engines what the image contains.
The writing formula: “Say in one sentence what’s in the image + (where it reads naturally) tie it to the page topic.”
| Scenario | Bad alt | Good alt |
|---|---|---|
| GSC screenshot | image1 |
GSC Page Indexing report showing 143 pages under Crawled - currently not indexed |
| Product photo | linen dress women summer slim korean(stuffing) |
Side view of an off-white linen dress, knee length |
| Flowchart | (blank) | Content-debt decision flow: consolidate, remove, or enrich |
The rules, boiled down:
- Content images must have alt; purely decorative images (dividers, background patterns) get an empty
alt=""(so screen readers skip them — that’s not the same as omitting the attribute). - Keep it to one sentence (roughly 5–15 words), not a paragraph.
- Don’t open with “an image of…” — the alt attribute already means it’s an image.
- The keyword appearing once, naturally, is fine; listing a string of keywords is a stuffing signal.
Move two: format
The 2026 cheat sheet:
| Format | Best for | Compression | Notes |
|---|---|---|---|
| AVIF | Photos, hero images (first pick) | Best (~50% smaller than JPEG) | Fully supported across mainstream browsers |
| WebP | The safe all-rounder | Good (~25–35% smaller than JPEG) | Most reliable compatibility |
| JPEG | Legacy compatibility | Average | Use as a fallback |
| PNG | Transparent backgrounds, UI screenshots | Poor (large files) | Swap to WebP wherever you can |
| SVG | Logos, icons, flowcharts | Vector, tiny | Text stays readable; first pick for diagrams |
Serve multiple formats with a <picture> fallback, or hand it to your framework (Astro’s <Image /> and Next’s next/image output AVIF/WebP automatically).
Move three: compression and loading
Dimension and size benchmarks:
- An image’s actual dimensions only need to be 1–2× its display size (Retina) — don’t drop a 4000px original onto the page.
- Target ≤ 150KB for content images, ≤ 250KB for the hero. Tools: Squoosh (free, runs in your browser), TinyPNG, or let your framework handle it at build time.
Three loading rules:
- Never lazy-load the above-the-fold hero — add
fetchpriority="high"instead. Lazy-loading the hero is the single most common way people slow down their own LCP. - Lazy-load everything below the fold with
loading="lazy". - Hard-code
widthandheighton every<img>so the browser reserves the space first and avoids layout shift (CLS).
Three easy wins people miss
- Semantic file names.
gsc-crawled-not-indexed-report.pngbeatsIMG_0042.png— the file name is a (lightweight) ranking signal. - An image sitemap, or image fields in your existing sitemap. Only sites that lean heavily on image traffic (e-commerce, recipes) need this; a typical blog can skip it.
- Original images beat stock. Custom flowcharts and real test screenshots double as first-hand E-E-A-T evidence; a stock photo seen everywhere has no search value.
FAQ
Q1: Will missing alt get me penalized? No penalty — but you’re leaving image-search traffic and accessibility points on the table. Treat alt as a field that should have been there all along, not an optimization trick.
Q2: How many images should an article have? There’s no magic number. The principle is that every image carries information. Two to five meaningful screenshots or diagrams in a tutorial beat ten decorative stock photos — the latter are just weight that slows loading.
Q3: Self-host images or use a CDN / image host? Host on your own domain with a CDN for speed. The risk of a third-party image host: if the service dies, all your images break, and traffic attribution for image search gets murky.
Q4: Do AI-generated images hurt SEO? Google doesn’t penalize images for being “AI-generated.” The test is still information value: an AI-generated diagram that accurately conveys a concept is fine; but passing off an AI image as a real screenshot in hands-on content damages E-E-A-T and reader trust.
GeoSeoToday treats images as “evidence carriers”: every screenshot testifies to the content’s first-hand experience — exactly what E-E-A-T and GEO both want. You can score how citable your article is overall with the GEO readiness checker.