I personaly don't know how exactly it is done but i do know that there is a panel that lets you select which channel to use.

My understanding is that you paste the image in as a layer, go to the tab that is for the channels (on the same control panel that you select the layer on, its the second tab, between layers and paths), turn off the RGB and select the Alpha chanel.

This is where my knowledge gets fuzzy, i think you can choose to select the area somehow and copy it as a mask or some such.

Hopefuly i led you in the right direction. If no one answers between now and the next 8 hours, i'll have a gander when i get home.