Push compression too far and you get visible damage called artifacts.
The three you'll see
- Blocking — the image breaks into 8×8 square tiles. Classic over-compressed JPEG.
- Ringing — fuzzy halos around sharp edges and text (worse with chroma subsampling).
- Banding — smooth gradients turn into visible stripes of color.
How to avoid them
- Don't drop JPEG quality below ~60% — see JPEG quality settings.
- Use WebP, which artifacts more gracefully than old JPEG.
- For flat graphics and text, use PNG instead of JPG.
The compress tool shows a live preview so you can stop before artifacts appear.