For snaps, sure, you can do that. But you need to create a mask image just right.
Basically, the mask needs to be exactly the same as the background, but the bottom left corner should be some weird color you don't use elsewhere in the image, and then everywhere where there's that color, will be transparent and allow the snap to show through.
Grab some Mala theme examples. There's lots out there that have that effect.
Also note that the masking DOES NOT WORK for videos (at least not yet, loadman would have to comment on that).
Videos are always square and always on top of everything else.
That's one of the reasons I've been working on Engine18 for Mala. It'll support masked videos (kind of does now, just can't handle rotation).