Identify effect for photos?

Hi All,

So I've got several photos I'd like to add an effect to - problem is I've no idea what this effect is called. Here is an example:

I basically want to create the black surround, not always black but the sort of jagged looking cut out effect with a photo behind it. Has anyone got idea what this is called or is there is a website you can get stock examples from? I've googling mask but I'm probably using the wrong terminology.

Many thanks to anyone who replies.