How to Optimise Images for WordPress: Complete Guide 2026

Images are the most common cause of slow WordPress sites. A default WordPress install with unoptimised images can have a PageSpeed score under 40. With proper image optimisation, the same site can score 85+. Here is exactly what to do.

Step 1: Resize Before Uploading

WordPress does not resize images to match your theme's display dimensions. If your theme shows featured images at 800×450px and you upload a 4000×3000px photo, WordPress stores the full-resolution file and serves it to every visitor. Always resize to the largest display size in your theme before uploading. For most blog themes, 1200×675px is sufficient. Use Image Resizer.

Step 2: Compress Before Uploading

A 1200×675px JPG at camera quality is typically 400–800KB. After compression at quality 80, it becomes 80–150KB with no visible quality loss on screen. Compress to 100KB before uploading blog thumbnails, or Compress to 200KB for hero images.

Step 3: Convert to WebP

WordPress has natively supported WebP since version 5.8 (2021). Convert JPG to WebP before uploading — you get 25–35% smaller files with the same visual quality. Your PageSpeed score will improve immediately.

Step 4: Use Descriptive File Names

• Before uploading, rename files to keyword-rich, hyphenated names:
• Bad: `DSC04521.jpg`
• Good: `wordpress-image-optimisation-guide.webp`
• WordPress uses the file name as the default alt text and image URL — both matter for SEO.

Step 5: Always Fill In Alt Text

Every image on your site should have descriptive alt text (5–15 words). Include the target keyword if it naturally fits. Never keyword-stuff alt text.

Step 6: Enable Lazy Loading

WordPress automatically adds `loading="lazy"` to images since version 5.5. Lazy loading defers off-screen images until the user scrolls to them — dramatically improving initial page load speed.