"J. P. Gilliver (John)" wrote

| PNG was I thought invented because some of the patents on some of the
| others were keeping some lawyers in business; those have expired now
| though. It also, being a more-recently-devised format, has some
| advantages. Whether it uses - or _can_ use - lossless compression, I
| don't know; some in this thread have said it is lossless, but haven't
| said whether that's optional. I don't originate it myself so don't know
| much about it, but sometimes I download images that are in it, and I
| don't like converting unnecessarily, so I have some PNGs.

As Rudy said, PNG is lossless, but it also has an alpha channel
for transparency. So it doesn't dump data the way JPG does,
and it also uses 4 bytes per pixel instead of the 3 that JPG
does. That makes it bigger. Each pixel can have a percentage
transparency, recorded in the 4th byte.

I don't know of anything to recommend PNG, but it could
make sense online where high image quality in 24-bit is needed.
There's no other widely supported formate for that. The
transparency feature is also nice, but I'm not aware of any
program for creating finely detailed transparency. For instance,
if you want a logo for "John's English Diner" and you want
a knife and fork semi-transparent, how do you do that in one
image? I don't know.