JPG or PNG
Short answer: photos go to JPG, screenshots, diagrams and anything with text go to PNG. Here is where that rule comes from.
| JPG | PNG | |
|---|---|---|
| Compression | lossy | lossless |
| Transparency | no | yes |
| 1920 × 1080 photo | 200–400 KB | 2–5 MB |
| Screenshot with text | halo around letters | clean edges |
| Re-saving | quality drops each time | file unchanged |
| Colours | 16.7 million | 16.7 million plus alpha |
Why a photo is so heavy as PNG
PNG is lossless: it hunts for repeats in the picture and writes them down more briefly. Flat fills and straight lines repeat a lot, so those files come out tiny.
A photo has no repeats at all. Neighbouring pixels of sky differ by a unit or two, which is sensor noise, and there is nothing there to squeeze. That is how one frame ends up at 300 KB as JPG and 4 MB as PNG while looking the same.
Why a screenshot looks bad as JPG
JPG works in 8 × 8 squares and coarsens fine detail. The hard edge of a black letter on white is precisely the fine detail it coarsens first.
You get a halo: grey mush around the text, and thin interface lines turn ragged. On a photo you would never notice; on a screenshot you notice immediately.
Transparency
Only PNG has it. Logos, icons, anything that has to sit on a coloured background — all PNG.
Convert to JPG and the transparent areas get painted over, white by default. If the page behind it is not white, your logo arrives with a white box around it.
What to do in practice
A shot from a camera or a phone: JPG. If the original arrived as PNG — messengers and editors love saving photos that way — convert it. The weight drops several times over and you will not see the difference.
A screenshot, a diagram, a drawing, a picture with text: PNG. It will be both lighter and sharper.
PNG too heavy and you do not need transparency? Cutting the number of colours or simply the pixel size sometimes helps, but more often the honest answer is JPG.