I ran across these sites today when I was thinking about pictures to use in a blog post. They are databases of free images; there are some restrictions of use – such as no “adult” usage, re-selling or using photos of people to endorse products – but for general use, even commercial use it’s OK to use these pictures:
Flickr Creative Commons pictures: http://www.flickr.com/search/advanced/?
With Flickr, you have to make sure to search for Creative Commons images, which means you can use them – but there are thousands available. You’ll just want to check to see if there’s an Attribution clause on the photo, which simply simple you should give credit to the photographer.