Image Compression Explained: Lossy vs. Lossless
Image Tools - 6 min read - Published 2026-09-06
Two image files can show the exact same photo and differ in size by a factor of ten or more, purely based on which compression approach was used. Understanding the difference between lossy and lossless compression explains why - and helps you pick the right format instead of guessing.
Lossless: Nothing Is Thrown Away
Lossless compression (used by PNG, and by ZIP files in general) finds patterns and redundancy in the data and encodes them more efficiently, but the original data can always be perfectly reconstructed - decompress it and you get back exactly what you started with, bit for bit. This works well for images with large flat areas of a single color, sharp edges, or repeated patterns (screenshots, logos, line art), where there's real redundancy to exploit. It works poorly for photographs, which have subtle, near-random variation in every pixel - there's little redundancy for a lossless algorithm to find, so the file stays large.
Lossy: Deliberately Discarding Information
Lossy compression (used by JPEG, and by WebP in its lossy mode) takes a fundamentally different approach: it deliberately throws away information the algorithm predicts you won't notice, based on how human vision actually works. JPEG specifically exploits the fact that human eyes are far more sensitive to changes in brightness than to changes in color - so it keeps brightness detail largely intact while compressing color detail much more aggressively. It also breaks the image into small blocks and reduces detail that's too fine to matter much at normal viewing size.
This is why JPEG compression can shrink a photo dramatically with only mild visible quality loss - it's not compressing blindly, it's using a real model of human perception to decide what's safe to discard.
Why JPEGs Get Blocky at High Compression
Push JPEG compression too far and you start seeing visible square-ish artifacts, especially around sharp edges and text. This is a direct visible consequence of the block-based approach described above - at extreme compression levels, the algorithm has discarded so much detail within each block that the blocks themselves become visible, and the smooth transitions between them break down.
Choosing the Right Format
Use lossless formats (PNG) for screenshots, logos, diagrams, and anything with text or sharp edges, where artifacts would be obviously distracting and file size savings from lossy compression would be modest anyway. Use lossy formats (JPEG, or WebP) for photographs, where the format's whole design assumption - lots of natural, gradual variation that compresses well and whose loss is hard to notice - actually holds true, and the file size savings are substantial.