Some features that would make this really useful:
- custom colors (as mentioned by others)
- an X across the image, corner to corner
- Optional text instead of the size (i.e. "lead photo", "thumb")
- a tag (or the resolution) repeated in an offset "brick" pattern across the entire image, similar color to BG
The last one probably overkill but useful for when you are doing something like scrolling window, where not all of the image is displayed.