I was recently told that png uses zip compression.. And as such it should be potentially lossless, as it compresses/uncompresses on the fly. Now I'm not sure.. I always save as tiff to get the alpha channel. Not png and not psd, though I do use them mainly in PS. Tiff works (as far as I know) on both PC, Mac and Linux.

FREEBIES! | My Gallery | My Store | My FB | Tumblr |
You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.