not 100% sure what your trying to achieve but,
maybe go around the edges using the eraser tool, with a soft brush, with a transperance of say 30%
this may work, or you could select around the image with the selection changed to have a 5 pixel shading thingy [located up the top - near where you can change the selection type]
hope that made sense
if not go here (go here anyway)
http://www.photoshop101.com/search.p...lending+images