The Real Cost of Compressing Images: Quality, File Size and Format Trade-Offs
October 1, 2026
"Compress this image" sounds like a single, predictable operation, but the same compression setting can be invisible on one photo and obviously degraded on another, because the underlying math responds very differently depending on what's actually in the image. Understanding why makes it much easier to judge whether a compressed file is actually good enough before you publish it.
What's the real difference between lossy and lossless compression?
Lossless compression (like PNG) reduces file size while preserving every original pixel exactly — decompressing gives back a bit-for-bit identical image, which is why lossless gains are typically modest, since there's a hard mathematical limit to how much truly redundant data exists in a file without discarding information. Lossy compression (like standard JPEG) deliberately discards information the algorithm judges least perceptible to human vision, which allows for dramatically larger size reductions but means some detail is permanently gone and can't be recovered by decompressing.
Why does the same compression percentage look fine on one photo and bad on another?
Lossy compression algorithms exploit visual redundancy — large areas of smooth, similar color (a clear sky, a plain background) compress extremely well with almost no visible artifact, while images with fine detail, sharp edges, or text compress much more visibly at the same setting, because there's less redundancy for the algorithm to exploit without discarding detail a viewer will actually notice. A portrait with skin-tone gradients tolerates more compression than a screenshot containing small text, which is why a single 'compress to 70% quality' setting can look perfectly fine on one image type and produce obvious blocky artifacts on another.
When should you use PNG instead of JPEG for a given image?
PNG's lossless compression and support for transparency make it the better choice for graphics with sharp edges, text, flat color areas, or any transparency requirement — logos, icons, screenshots, diagrams. JPEG's lossy compression is built for and excels at photographic images with continuous color gradients and natural detail, where its size savings are dramatic and the discarded information is genuinely hard to perceive. Using PNG for a photograph produces an unnecessarily large file for little quality benefit; using JPEG for a logo or screenshot with sharp text edges often introduces visible artifacting around those edges that PNG would have avoided entirely.
What is WebP, and is it actually worth switching to?
WebP is a more modern format supporting both lossy and lossless compression, generally achieving smaller file sizes than JPEG or PNG at equivalent visual quality, plus support for transparency and animation in a single format. Browser support has become close to universal, which removed the historical objection to using it. The main remaining friction is workflow — some older tools, email clients or specific platforms may not handle WebP as smoothly as the older formats — but for web delivery specifically, it's now a genuinely strong default rather than an experimental choice.
How do you actually judge whether you've compressed an image too much?
Visible artifacting is the clearest sign: blocky patches in areas that should be smooth gradients, a slightly blurred or 'muddy' look to fine detail, or visible banding in what should be a smooth color transition. A useful practical test is viewing the compressed version at the actual size it will be displayed (not zoomed in past 100%, which exaggerates artifacts irrelevant at normal viewing size) — an image can show compression artifacts under close inspection while looking completely fine at its actual display size on a web page, and optimizing for the zoomed-in view wastes file size the real viewer will never benefit from.