A simple way to create a grunge like backround, if I got that expression right.
Use a canvas of 2400*1800 pixels for this.
Make clouds with black and white (settings: Scale 850, Roughness 1.00).
Create a new layer and ctrl+f.
Change the top layer to xor.
Do fragment with default settings on the top layer.
Optional: Flatten and Gaussian blur with setting of 1.
I don't know how to do thumbnails, I'll just link it.
http://img297.imageshack.us/img297/7379/grungerq2.png