aint every sig you do with a sig is a manip? you cut out the render from its bg, and add your bg, which you have a "manip"..... blah forget my babbling.
I use stock photos from deviantart for my manips.....also I use sxc.hu, imageafter.com, and morguefile.com. To use da stock outside the site (note that I said stock, not deviations, there's a difference) you have to get permission, but the others are all license-free, you can do what you wish with them.