Optimisation d’images pour le SEO : Formats, Compression et Alt Texte

Images account for the largest portion of page weight on most websites — often 50-80% of total bytes. Optimizing images is the single most impactful action you can take to improve page speed, Core Web Vitals scores, and search engine rankings.

Modern formats matter: WebP provides 25-35% smaller files than JPEG with equivalent quality, and browser support is now universal. AVIF goes further with 50% savings, though encoding is slower. For maximum compatibility, serve AVIF with WebP and JPEG fallbacks using the <picture> element.

Compression should be aggressive but intelligent. For photographs, JPEG quality 75-85 or WebP quality 75-80 provides excellent results with minimal visible degradation. For screenshots and diagrams with sharp text, PNG or WebP lossless gives pixel-perfect rendering.

Properly sized images are crucial — never serve a 4000×3000 pixel image when the display size is 800×600. Use responsive image srcset attributes to serve different sizes at different breakpoints. This alone can reduce image bytes by 60-80%.

ConvertCraft's Image Converter helps with bulk format conversion and compression — convert entire folders of JPEGs to WebP, resize batches to specific dimensions, or strip EXIF metadata for privacy. All processing happens in your browser with zero upload.