Image Optimization & Alt Text: The Accessibility SEO Guide

Why Image Optimization is Critical for SEO

Images serve three SEO purposes:

1. Google Images ranking: Optimized images appear in Google Images search (10-30% of organic traffic for visual topics)
2. Page speed: Uncompressed images slow your site, hurting Core Web Vitals (ranking factor)
3. Accessibility: Alt text helps screen readers (ADA compliance, ranking signal)

Image optimization is a key component of complete on-page SEO—learn the full strategy in our comprehensive on-page SEO guide.

The Cost of Poor Image Optimization

• Unoptimized images (2MB+ each) = LCP (Largest Contentful Paint) > 4 seconds
• Slow page speed = ranking penalty (Google Page Experience)
• No alt text = Google can't index image content
• Poor filenames = missed search visibility

Alt Text Best Practices: Keyword + Description

What is Alt Text?

Alt text (alternative text) is a short description of an image. It appears when:

• Image fails to load (broken link)
• User hovers over image (some browsers)
• Screen reader reads page (accessibility)
• Google indexes image content (Google Images)

Alt Text Formula: Keyword + Description (125 chars max)

Structure: [Keyword] + [Descriptive context]

Good alt text: "Internal linking strategy diagram showing pillar page linking to cluster posts"

Bad alt text: "image123.jpg" or "diagram" or "Internal linking strategy diagram for South African SEO businesses with keywords"

Alt Text Guidelines

• Include target keyword: If the image is about "title tag optimization," include "title tag" in alt text
• Be descriptive: Describe what the image shows, not just the topic
• Keep it under 125 chars: Google recommends short, punchy descriptions
• Don't stuff keywords: "title tag title tag SEO title tag optimization" = bad
• Use punctuation: "Keyword: description" or "Keyword - description" reads better

Alt Text Examples by Image Type

Diagrams/Charts: "User search journey chart: awareness → consideration → decision stages"

Screenshots: "Google Search Console CTR report showing 2% average click-through rate for homepage"

Infographics: "SEO ranking factors ranked by importance: backlinks, content quality, page experience"

Decorative images: Leave blank (alt="") — don't waste alt text on pure decoration

Image Compression: Balancing Quality & Speed

Target File Sizes (by format)

• JPEG: 50-150KB per image (lossy compression acceptable)
• PNG: 100-300KB (lossless, larger files)
• WebP: 30-100KB (modern, best compression)
• SVG: 5-50KB (scalable, ideal for logos/diagrams)

Compression Tools

• Free: TinyPNG, ImageOptim, Squoosh (Google's tool)
• Paid: Cloudinary, ImageEngine (auto-compression on delivery)
• Built-in: WordPress Smush, Yoast SEO (auto-compress on upload)

Filename Optimization: SEO-Friendly Naming

Good Filename Format

Formula: keyword-descriptor-format.ext

Good: "title-tag-optimization-guide.jpg"

Bad: "image123.jpg" or "screenshot (1).png" or "photo-123-final.jpg"

Filename Best Practices

• Use hyphens to separate words (not underscores or spaces)
• Include 1-2 keywords if relevant (not forced)
• Use lowercase letters only
• Keep it under 50 characters
• Match filename to image context (improves indexing)