When you go to the channels tab, you can click on the tiny 'document' icon, you get an empty alpha channel. Click on that and paste the source image (that also should be the same size as the target file). If you make the picture AND the extra channel visible, you get to see a kind of a preview to the end result, everything that is black will be masked/gone later. You can resize or filter or distort or whatever the alpha channelin any way you like. That is one of the main reasons to use an alpha channel instead of a mask. I hardly use any of those pre-made Edge FX things any more, I bake my own borders! You can make a selection of an alpha channel (option-click on the channel or use 'make selection'). With that selection you can make a new layer once you click on the layer or add a mask to the layer the picture is in (it should have been double-clicked, not be a background layer anymore, but Layer 0 or something). Have fun!