To clarify, 32 bit PNGs can have partial transparency. Images could use differing number of bits, either 8-bit (256 colors, one transparency channel), 24 bit (true color, no transparency), or 32 bit (true color with full transparency abilities). Perhaps the transparency could be controlled by the user, so they could turn it fully transparent to turn it "off", fully opaque for the default, true image, 50% transparent, etc...