my best guess is: alpha channels
use the channel tab (next to the layer tab)
select the parts you want to be visible (magic wand tool), then just like you make a new layer in the layer tab make a new channel, this will be the alpha channel and only the white selection thing (after you create it) is what's gonna show
try playing around with that, I'm pretty sure that's how you do it, but it may be the opposite (chooose what you don't want and make a channel) anyways i hope this helps