PNG to JPG

PNG stores every pixel exactly, with no discarding of detail, which is exactly right for graphics but expensive for photographs — a photo has continuous gradients and subtle noise everywhere in the frame, and losslessly encoding all of that leaves little room for compression. JPG's lossy model was built specifically for this kind of content: it identifies the high-frequency detail human vision is least sensitive to and discards it first, which is why the size drop on a genuine photograph is dramatic rather than incremental — commonly 80–90%, not 20 or 30%.

The one universal cost of the conversion is transparency. JPG has no alpha channel at all, so any transparent or semi-transparent pixel in the source PNG has to be resolved to an opaque colour before the file can be saved as JPG — there is no setting that preserves it, because the destination format has nowhere to put it. Compositing onto white is the sensible default for nearly every real use case; the alternative, leaving those pixels undefined, is what produces the solid black squares people sometimes see from other tools.

The size saving that makes this conversion worthwhile for photographs works against you for flat, high-contrast content. A PNG of a logo, an icon, a screenshot of text, or a simple illustration is often already close to its practical minimum size, because it has few distinct colours and little of the gradient detail JPEG is built to discard. Converting that kind of image to JPG frequently produces a smaller file that also looks visibly worse — soft edges around text, faint haloing around hard boundaries — for a saving that is proportionally much smaller than what a photograph would give up.

The distinguishing question is simply whether the image is photographic. If it came from a camera or contains realistic gradients and lighting, JPG is very likely the better shipping format. If it was created in a design tool, a screenshot utility, or contains text and flat colour, PNG usually remains the right choice even at a larger file size.

Quality 90 is a reasonable default for most photographic PNG-to-JPG conversions because it sits comfortably above the point where JPEG's compression artefacts become visible, while still capturing most of the available size saving — the difference in file size between quality 90 and quality 100 is often substantial, while the visible difference is close to none. Dropping to quality 75 or 80 typically shrinks the file further still and remains acceptable for casual viewing, but is worth avoiding for an image that will be printed, zoomed into, or edited again afterwards.

The one case worth setting quality noticeably higher than 90: a PNG screenshot containing small text that is being converted to JPG only to satisfy an upload requirement. Text edges are exactly where JPEG artefacts show up first, so a setting that looks fine on a photograph can make small lettering visibly fuzzy at the same number.