What I noticed, if you want to bring transparancy with your png you should make 24bit-pngs, I made some 8bit ones and the transparancy wouldn't come in. But perhaps that was a bug in the last version and is now fixed.
The first 24 bits are the image information (3x8bits R,G,B), and the alpha channel is stored in the extra 8bits that make up the 32-bit PNG file.
Google RGB and RGBA for more info.
Krush.