I have a number of Flickr galleries I use for NPC portraits. You can see them all here:
http://www.flickr.com/photos/94313484@N00/galleries/I posted a couple of these before and, as Meg mentioned, many of the photos are not released under any kind of license. My personal feeling is that using them for a private campaign is kosher, though they have to be referenced via the galleries since you can't obtain the files for print-outs.
I maintain a separate (but overlapping)
Picasa webgallery of photos that were available for download -- unfortunately they are consequently stripped off their Flickr-attribution -- and I make (relatively low-quality) printouts of these to use at the table.
As Johnstone has emphatically repeated in the thread on tracking NPCs, a good photograph really does go a very long way. For example, the moment I showed
this photo for Pimple, a local hardholder, he was pretty much guaranteed mental immortality. He hasn't showed up the last few sessions, but I am confident that even if he doesn't appear for another six sessions, none of the players will have trouble remembering who he is.