Image SEO 2025 — Practical alt text, structured data, and sitemaps

Published: Sep 18, 2025 · Reading time: 3 min · By Unified Image Tools Editorial

Introduction

The days of “just place an image and it will be picked up” are over. To fully capture search traffic you need meaning (alt/filename), context (structured data), discoverability (image sitemaps), and speed (LCP optimization) all in place. This article summarizes a minimal, practical setup you can adopt with confidence in 2025.

TL;DR

• Alt should let the text stand on its own without the image. Decorative images use empty alt; informative images are specific.
• File names should be alphanumeric + hyphen and describe the content. Avoid names like img001.jpg.
• Use structured data (Article + ImageObject) to strengthen meaning around key images.
• Maintain an image sitemap to stabilize indexing.
• For LCP candidates, eliminate overserving with resize/srcset/sizes, and control priority.

Alt text — write for “the reader who can’t see”

Alt is a substitute for screen readers and when images don’t load. Summarize in one sentence the role the image plays in the paragraph. Decorative separators or pure background visuals can have alt="" (empty). For images that carry information (diagrams, comparisons, steps), include proper nouns and numbers—be specific.

Good examples:

• A slider comparing pre/post-crop images (the Before/After difference is obvious)
• “Input example for the print size calculator (width 210mm, 300 DPI result)”

Bad examples:

• “image1,” “photo,” “image”
• Keyword stuffing (unnatural, spammy)

File names — align search intent and human understanding

• Use alphanumerics + hyphen to briefly denote “what it is” (e.g., webp-compression-example.jpg).
• Avoid spaces/Unicode/symbols; make sure the extension matches the actual format.
• For series, use numeric suffixes like -1/-2 without breaking semantic chunks.

When renaming existing assets, combine with fingerprinting for stable caching. For bulk operations, use Bulk Rename & Fingerprint.

Structured data — reinforce meaning with Article + ImageObject

For primary images tied to an article, add an ImageObject in structured data. The following fields are practically useful:

• url / width / height
• caption (figure description)
• representativeOfPage (whether it’s the representative image)

On this site, we integrate OG/Twitter images into the article’s JSON-LD (Article) and list each as an ImageObject (see implementation in the codebase).

Image sitemaps — guide crawling and canonicalization

You don’t have to list every single image, but maintaining sitemaps that enumerate key distribution paths (articles/categories/tools) with image:image improves index stability. Generate diffs on updates and clean out 404s.

Automate generation. Extend scripts/generate-sitemap.mjs in this repo to emit <image:image> entries.

LCP candidates — design from layout backward, stop overserving

For LCP candidates (hero or early figures), design in this order:

1. Compute pixel upper bounds from container width × expected DPR
2. Prepare 3–5 representative widths and write proper srcset/sizes
3. Use priority/preload/fetchPriority to boost initial paint

If unsure where to start, see Right-Size Images in 2025 — Layout-Backed Resizing That Cuts 30–70% Bytes and Responsive Images in 2025 — Srcset/Sizes Patterns That Actually Match Layout. Visualize overserving first to avoid mistakes.

Operational pattern — automate to prevent drift

• Keep a high-quality master; generate derivatives in a single build pass
• Use WebP as a baseline + validate AVIF; unify ICC/metadata policy
• Long-term cache + fingerprinting to avoid breakage on replacement

Helpful tools to combine:

• Srcset Generator: Draft srcset/sizes
• Format Converter: One-off conversions + quality checks
• Image Resizer: Determine upper bounds from layout

Summary

If you design around four pillars—meaning (alt/filename), context (structured data), discoverability (sitemaps), and speed (LCP)—you can grow search traffic without keyword stuffing. Start by unifying policy for primary images in existing posts, then cut overserving for LCP candidates.