if you want to make a border out of a picture (or a specific layer) you can do any of the suggestions above (i usually just make the shape white) and place it under the layer i want to show as the border. Move to the layer palette and hold down the ALT key with the cursor "between" the two layers and you should see an icon that looks like a half filled circle. the one layer should indent in the layers palette and theone layer should transform into the shape you made for the border.

 

scott