AAAgggghhhhh! I've been trying to remember how to do this for hours.

What I want to do: I have two photo files. One is a portrait, the other is a grunge shot. I want to use the grunge shot as a border for my portrait, and I can't work out how to do that. I want to be able to see the portrait "through" the border as well.

Last year I figured out how to do it using some mask technique, but do you think I can remember how to do it?