Never underestimate the potential of Google-Fu, young one.
DeviantArt is a good one, as darkangel mentioned. You'll find a good mix of artwork and normal pictures there. If you want something less artsy fartsy and more straight up pervy, Gelbooru is great for artwork, though it's mostly anime-style, with the exception of if the picture is from a certain fandom.
It really varies greatly on what sort of pictures you'd be interested in using. Real or drawn? Alluring/suggestive/softcore or lewd/hardcore? Etcetera.