JPG vs PNG: which one to use, and when converting helps

JPG and PNG are both thirty years old and both are everywhere, which is why so many uploads fail on the difference between them. The short version: JPG is for photographs, PNG is for anything with sharp edges or transparency, and a form that names one of them means it.

What each one is good at

JPG compresses photographs by discarding detail the eye does not track: subtle colour shifts, fine grain. The result is small, and on a photo it is invisible at sensible quality settings. On text, a logo or a line drawing the same process leaves a faint smudge around every edge, called ringing.

PNG stores pixels without loss. On a screenshot full of flat colour it is small, often smaller than JPG, and perfectly crisp. On a photograph it is large, because there is no repetition to exploit: a 12-megapixel photo is 20 to 30 MB as PNG and 3 MB as JPG.

Transparency

PNG has an alpha channel; JPG does not. Converting a logo with a transparent background to JPG fills the transparent area with a colour (white, on this site, unless you choose another). If the transparency matters, keep PNG or use WebP.

Typical sizes, same image

Figures are typical, not exact; your file will differ. The pattern holds: photos want JPG, flat graphics want PNG.

When converting helps

• A form says JPG only and you have a PNG: convert, and check the size afterwards.
• A screenshot is being rejected for size: converting to JPG at quality 90 usually halves it with text still readable; WebP does better if accepted.
• You will edit a JPG repeatedly: convert to PNG first, so each save does not add another round of loss.
• A scanned signature or document: PNG keeps the strokes clean; convert to JPG only if the portal requires it, at quality 90 or above.

What converting cannot do

JPG to PNG does not restore detail the JPG already lost, and PNG to JPG does not keep transparency. Converting changes the container and the compression, not the information that is left.