What Image Compression Does
Image compression reduces file size by removing data the viewer is unlikely to notice. An uncompressed photograph from a modern camera can be 5–10MB. Proper compression brings that down to 100–300KB at the same visual dimensions — a 95% reduction — with minimal perceptible quality loss.
For WordPress sites, compression is one of the simplest performance wins available. Images are typically the heaviest assets on any page, and compression reduces their weight without changing how they look, how they are delivered, or how they are coded into the page.
Lossy vs Lossless Compression
Lossy compression permanently removes visual data that the human eye is unlikely to detect. JPEG compression is lossy — it discards high-frequency color information. A JPEG saved at quality 80 is typically 60–70% smaller than quality 100, with differences only visible under close magnification. For photographs and complex images, lossy compression is the right choice.
Lossless compression reduces file size without removing any data — the decompressed image is pixel-identical to the original. PNG uses lossless compression. The savings are smaller (typically 10–30%), but no information is lost. Lossless is appropriate for graphics, logos, screenshots, and images where every pixel matters.
Quality Settings and Diminishing Returns
JPEG quality is measured on a scale from 0 to 100. The relationship between quality number and file size is not linear. Dropping from quality 100 to 85 typically saves 50–60% of the file size with almost no visible difference. Dropping further to 70 saves another 20–30% but artifacts start becoming noticeable on detailed images. Below 60, compression artifacts are visible to most people.
For most WordPress use cases — blog images, product photos, hero images — quality 75–85 is the practical sweet spot. This range delivers substantial file size savings while maintaining visual quality that visitors and clients will find acceptable.
WordPress Default Compression
WordPress applies some compression when generating image sizes on upload. The default quality is 82 for JPEG images (lowered from 90 in WordPress 4.5). This is reasonable but not aggressive — many images uploaded to WordPress could be further compressed by 30–50% without visible quality loss.
More importantly, WordPress only compresses the derived sizes it generates (thumbnail, medium, large). The original uploaded file is stored uncompressed. If your theme or page builder serves the full-size original — which some do — visitors download the uncompressed version.
Tools That Can Help
ShortPixel compresses images on upload with adjustable quality levels (lossy, glossy, and lossless modes). It also bulk-optimizes existing media libraries and can compress the original uploaded file — not just the WordPress-generated sizes.
Imagify offers three compression levels (normal, aggressive, ultra) and integrates with WP Rocket for coordinated optimization. Its visual comparison tool lets you preview compression results before committing.
EWWW Image Optimizer processes images on your own server rather than sending them to an external service. This is useful for sites with large media libraries or privacy requirements, though it requires more server CPU during optimization.
Further Reading
- Optimize and compress images (web.dev) — Google’s guide to image compression techniques, tools, and quality trade-offs.