Morgue Files is my favorite place to look for free online photos.
No registration or account needed. Attribution info clearly posted.

Watch the video below for a quick tutorial that covers
the basics of searching, downloading, and using the pictures available.

Click here to link to a larger version of this same video.

Other online tutorials/info:

http://www.youtube.com/watch?v=ws2VfY6UezI&feature=player_embedded#!