Croquis Cafe is an online art resource with a focus on classic painting and drawing education. They have been kind enough to put hundreds of model photos online for free with a pretty generous license.
The photos are not that high resolution and can occasionally be quite grainy, but what makes this library stand out from many places is the variety of body types.
Please note that I redacted the explicit nudity of the samples above, while the actual photos in the gallery are unredacted.
You can get the stuff here: http://www.onairvideo.com/croquis-cafe-photos.html