Almost every image you save asks you to choose a format, and most people pick whichever came up last. That choice quietly decides whether your file is 200KB or 2MB, whether it keeps a transparent background, and whether the text in a screenshot stays sharp. The rule is simpler than it looks.
The one-line rule
Photographs go to JPG. Anything with sharp edges, flat colour or transparency goes to PNG. A holiday photo has millions of subtle colour shifts, which is exactly what JPG compression handles beautifully. A logo, a screenshot with text, or an icon has hard edges, which JPG blurs into fuzzy smudges but PNG stores perfectly.
What each one costs you
- JPG: far smaller files, but no transparency and a little quality lost each time you save.
- PNG: perfect quality and full transparency, but files can be many times larger.
- WebP: usually beats both, offering small files with transparency, supported by all modern browsers.
Saving the same JPG over and over slowly degrades it, because each save re-compresses an already compressed image. Always edit from the original file rather than a copy of a copy.
Converting between them
If you have a large PNG that is just a photo, converting it to JPG will usually shrink it dramatically with no visible difference. Going the other way, from JPG to PNG, stops any further quality loss but will not recover detail that has already gone. Both conversions take seconds and run entirely in your browser, so your images are never uploaded.