PNG is lossles, but, the editing tool may use palette with the fixed colors to generate a PNG. Almost always PNGs do not use the palette, but, still there's an option for it, I can imagine, that if you'll have a file with only few different colors, than the editing tool may automaticaly switch to use the palette (you'll have to check the input in another, more advanced editor to make sure the PNG was saved exactly as you wanted).
Also, if you transformed the image and used BitmapData.draw() on the transformed one, than there may be difference.
Oh, and by thee wee %) Have you saved your PNG as 8 bits per channel or 16 bits per channel?
Last edited by wvxvw; 09-18-2008 at 06:16 PM.