Well, I use GIMP, but maybe it's the same.
Create a new layer and select all. Then go select shrink, and put in about 5. Then go Select>>Invert and you can fill it with black or whatever.
Or
If you have a picture, let's say, in the middle of a blend. Make sure that picture is on a seperate layer. Create a new layer, between the image layer and the background layer. Take the rectangle selecter and make a selection just around the image and fill it wih black...Tada! You're done!
Hope this works for you
