When you must keep PNG — for a logo, icon, or screenshot with transparency — this optimiser re-encodes it with maximum lossless compression, trimming redundant data while keeping every pixel and the alpha channel. For photographs, JPEG compression will shrink far more.
When PNG is the right call (and when it isn't)
PNG earns its keep on anything with flat colour, sharp edges, or transparency: logos, UI icons, line art, charts, and screenshots of text. Its lossless compression keeps those crisp boundaries pixel-perfect, where a JPEG would smear them with blocky artefacts. The catch is photographs. A continuous-tone photo has millions of subtly different pixels, and PNG has to record nearly all of them, so the file balloons. If your compressed PNG is still several megabytes, that is the format fighting the content, not a flaw in the optimiser. The fix is to switch formats: send photographic material through JPEG compression and reserve this tool for graphics that genuinely need exact pixels and an alpha channel.
What lossless PNG optimisation removes
Because nothing visible is discarded, the savings come from trimming overhead rather than detail. The optimiser re-derives the best filter for each row, picks an efficient palette where the image uses few colours, and rebuilds the compression stream more tightly than most editors bother to. It also drops bulky ancillary chunks — embedded colour profiles, thumbnails, and editing metadata — that add bytes without changing how the picture looks.
- Indexed colour: icons and flat graphics often collapse to a small palette, shrinking dramatically.
- Better filtering: screenshots with large solid regions compress especially well.
- Cleaner stream: redundant data is squeezed out without touching a single visible pixel.
Combine PNG optimisation with the right next step
One pass through this optimiser usually gets a graphic close to its lossless floor, so running it repeatedly won't help much. To go smaller you have to change something about the image. If the PNG is also larger on screen than it needs to be, scale it first with the image resizer — fewer pixels means a smaller file before optimisation even runs. If transparency isn't essential and you can target modern browsers, WebP typically beats optimised PNG by a wide margin while still supporting an alpha channel. For everything else, compressjpeg.onl keeps these tools one upload apart so you can try each without losing your place.