Photoshop :: Brush For "bite" Marks? And Trans Edges.

I just want a brush for PSCS that I can run across an image to look like someone, or something, was biting the edge.

It doesn't have to be fancy or super "real" looking. Just better looking then using a round eraser and chopping it up. Or a "leaf" brush or some other thing.

I also have another question. It's probably quick to answer so I'll just ask it here too. When layering photos over the top of each other; what is the best way to do gradient transparencies around one image, so the image below shows through and looks nice?

I try using just the eraser with a dropped down opacity, but there is no smoothness to the transparency. I tried erasing with a feathered selection, but that was erasing the main part of the "top" image along with the edges.

Whatever technique it is, it would have to be useable on a round image, or square, or utterly hacked up shape. Simply so it has a smooth transparency to the image below.
I can do this easy with a transparent gradient, IF the bottom was just a solid color.