Kind of a tangent, but while the png format uses lossless compression, you do not know that the .png you have in front of you is lossless from the filename; image softwares can and do support preprocessing an image with lossy algorithms to improve the file size.

This is sort of unlike flac for audio -- it is technically also possible, but hardly anyone would do it, since the only reason to use flac is for lossless compression.

I get the impression that .png is used alongside .jpg as a general image format, although i don't really know.

You also don't know if your .png file has an alpha channel or not -- might have been made without one.