Alpha channels have nothing to do with resolutoin.  If you have a more advanced photo editor, you can see channels (usually a red, green, blue channel and an alpha - Alpha are great to use for making image masks)
pngs (sometimes)
psd
tif
formats all can have an alpha channel.  Poser 7 is not creating one like it should be if there's a background picture - Poser 6 makes them just fine.  (I havent tried tiff yet - but that's a work around I may have to do for now)

Doug

I came, I rendered, I'm still broke.