This is an outdated thread, but I just encountered the problem and would like to share my solution. Just in case somebody runs into the same problem.
Actually when it says "Failed to load image "IMAGE NAME". Reason : PNG not supported: 8-bit only" It actually meant it only accepts 8 bit.
The admin kept saying it doesn't accepts 8 bit, which confused a lot of people. But it's actually the opposite: it only accepts 8 bit. I don't know about 24 bits, but I think 16 and 32 does not work.