Choose PNG for transparency or sharp edges
PNG keeps a transparent background, which JPG cannot store at all, and it keeps hard edges clean, which JPG blurs. Those are the reasons to pick it. It is not the reason most people think: PNG will not improve a WebP's quality, and the file will be considerably larger.
Lossless preserves. It does not restore.
"Lossless" sounds like a quality setting. It is not — it is a promise about the conversion, not about the picture. PNG guarantees it will store precisely what it was handed. What it is being handed here is an image WebP already compressed, with whatever softness and blocking that introduced baked in.
So the PNG is a perfect copy of an imperfect picture, at several times the size. That is a fair trade when you need the transparency or the sharp edges. It is a bad trade for a photograph you just want to send someone — convert that to JPG instead.
What happens to the file size
| What the image is | PNG versus the WebP | Why |
|---|---|---|
| Logo, icon, flat colour | Somewhat larger | PNG compresses repeated exact colours well, though lossless WebP usually does better. |
| Screenshot with text | Somewhat larger | Mostly flat, but the text detail costs. |
| Photograph | Several times larger | Photographs have few exact repeats, so PNG has little to exploit. |
If the result is too large to use, that is PNG working as designed rather than a fault. The compressor explains what can and cannot be done about a PNG, and resizing is the one lever that always works.