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Two formats, one question, and a surprising amount of bad advice. The rule is simpler than people make it — and there is one specific case where choosing wrong turns your background black.
There is a lot of confident nonsense written about this, most of it some version of "PNG is higher quality". That is true in a narrow technical sense and misleading in every practical one. Here is the actual rule.
Does the image have large areas of flat, identical colour, or sharp edges, or transparency?
That is genuinely it, and it will be right almost every time. If you want to know why, the reason is quite satisfying.
JPEG was designed for photographs, and it exploits something specific about human vision: we are far more sensitive to changes in brightness than to changes in colour, and we barely notice fine detail in busy areas. So JPEG throws away the detail we are worst at seeing, and keeps what we notice. On a photograph of a face, or a landscape, or a plate of food, this works extraordinarily well — you can discard ninety percent of the data and most people cannot tell.
PNG takes the opposite approach: it finds repetition and encodes it compactly, and it never discards anything. A screenshot with a large white background compresses beautifully, because "3,000 identical white pixels" is a very short thing to write down. And when you open it again, every pixel is exactly what it was.
Use each on the other's material and both fall apart.
There is almost no repetition in a photograph — every pixel differs slightly from its neighbour. PNG cannot find anything to compress, so it stores nearly all of it. The same holiday photo might be 800KB as a JPG and 6MB as a PNG, and you would not be able to tell them apart on any screen.
This is the worse direction, and you can see it immediately. JPEG works in 8×8 pixel blocks and smooths within them. Around sharp black-on-white text, that smoothing produces a faint grey mist — "ringing artefacts". Screenshots saved as JPG look dirty in a way that is hard to name but easy to notice, and every re-save makes it worse.
This is the one that catches people, and it is worth knowing before it happens.
JPEG cannot store transparency. There is no mechanism for it in the format at all.
So when you convert a PNG with a transparent background to JPG, those transparent pixels have to become something. What they become depends on the tool — and a naive one leaves them black, because the underlying canvas starts as transparent black and the transparency is simply discarded.
You export a logo, and it arrives with a black rectangle around it.
Any decent converter fills the transparent areas with a colour you choose first. Our PNG to JPG converter defaults to white and lets you pick, for exactly this reason. But the underlying lesson is simpler: if the image needs a transparent background, it cannot be a JPG. No setting changes that.
When you save a JPG you choose a quality level. Some notes worth having:
So if an image is still being worked on, keep it as PNG and export to JPG once at the end.
WebP does both jobs and does them better — typically 25–35% smaller than JPEG at the same visible quality, and it supports transparency. Every current browser handles it.
Use it for anything going on a website. Be more careful elsewhere: some older software still refuses it, and a few printers and office applications will not open one. If you are sending a file to another person rather than publishing it, JPG and PNG remain the safe choices.
| What it is | Use | Because |
|---|---|---|
| Photograph | JPG | Far smaller, no visible loss |
| Screenshot or diagram | PNG | Sharp edges stay sharp |
| Logo or icon | PNG | Transparency survives |
| Anything with text in it | PNG | No ringing around letters |
| For a website | WebP | Smaller than both |
| Still editing it | PNG | Re-saving costs nothing |
If you need to move between them, the converter and the resizer both run in your browser — the file never leaves your device.