QPixmap::save() has a quality setting for images, but it does not distinguish between lossy and lossless compression. For PNG the lowest quality setting is the best (lossless) compression and the highest setting produces huge seemingly uncompressed files. For WebP the quality controls lossy compression, except 100 (the highest setting) is lossless.

It is impossible to pick a quality setting if I don't know beforehand which image format I'm saving. There should be 2 settings, 1 for lossy compression and 1 for lossless compression. Or a higher quality could mean a higher lossless compression.

Judging by the documentation it looks like it is the same in Qt 6, but I haven't verified it.