or you could make a brush...and just heavily fsck it up....
basically...take some photo......run a very heavy threshold (image > threshold) on it, then select either the white or black and delete, then add some random noise and stuff to it, reduce the opacity to 35% or something, ctrl+a, edit>define brush
grab your paintbrush and have some fun
this is good too cuz it's very re-useable and transferable