Sometimes if I upload certain images (mostly PNG, I think) file manager doesn't seem to make the thumbnail properly, which would be OK but it puts a massive default image (blank, or with flowers...) in to the thumbnail space instead.

The attached pic shows what I mean.

It's kind of ugly and takes up loads of screen...
Is this meant to happen? And if there isn't a way around, does someone know where these default images come from and whether they can be shrunk to a sensible size to match the standard thumbnails?

Thanks