Why JPGs Degrade: How Compression Artifacts Stack
Edit and re-save a downloaded JPG a few times and the picture turns "dirty": fuzzy edges, broken gradients, mesh patterns in flat areas. Not mystique — JPG's lossy artifacts accumulating generation by generation.
The three faces of artifacts
JPG compresses the image in 8×8 blocks, and three classic artifacts trace back to it: blockiness — block borders become visible at high compression, like a faint grid over the picture; mosquito noise — fuzzy fringes around object edges; banding — smooth gradients break into stepped stripes, most visible in skies.
All three exist from the first save, just invisible at high quality settings. The problem: every re-save re-quantizes the whole image, and last round's artifacts are treated as real content in the next round. Errors compound, and a few rounds later the damage is plain to see.
When it worsens and how to avoid it
The danger list: repeated "save as JPG" in editors, multiple forwards through chat apps (each may re-compress), and screenshots of screenshots. Every pass adds a layer.
The right route depends on the stage: use lossless formats like PNG throughout editing — format conversion costs nothing; export JPG exactly once at the end, with quality around 85 to 90 — lower shows artifacts, higher wastes size. Already-degraded images cannot be repaired: the discarded information is not in the file. Reshoot, or reconstruct via upscaling (see the photo upscale guide).