PNG vs JPG: which one should you save as?

Everything else follows from lossless versus lossy. A PNG decoder returns the exact values the encoder was given. A JPEG decoder returns an approximation whose closeness depends on the quality setting. Neither is better in general; they are built for different pictures.

On a photograph, the approximation costs you almost nothing you can see, because the discarded information is mostly fine texture and sensor noise, and it saves you most of the file. On a screenshot, the same discarded information is the crispness of every letter and line, and you see the loss at once. That is the whole rule, and the decision table below is just that rule applied to common cases.

Two practical constraints settle many cases before file size even comes up. First, JPG has no alpha channel. A logo, sticker or product cut-out with a see-through background must stay PNG (or move to WebP or AVIF); saving it as JPG fills the background with a solid colour. Second, PNG survives repeated saving untouched, while each lossy JPG save re-quantises the image. If a file will be opened, changed and saved many times, keep it as PNG until the final export.

The reverse conversion is a common trap. Turning a JPG into a PNG does not restore anything the JPG discarded: the PNG faithfully preserves the JPG's artefacts and is usually much larger. Convert JPG to PNG only when the destination demands PNG or you are about to add transparency.

Upload forms, especially government and exam portals, often accept only JPG and also cap the size in KB. A screenshot of a document is then better converted to JPG at a high quality and checked by eye than rejected as a PNG. When a form accepts both, compare the two files' sizes for that particular image and keep the smaller one that still looks right.