Well, there are a few places I know of where you can get free images.

Flickr.com:

After you do a search you can click the 'advanced' link and scroll down and select the check box that says: 'Only search within Creative Commons-licensed content'. Be forewarned some of the stuff may posted under a creative commons license, but that doesn't necessarily mean it should be. For example, I was looking on there the other day and saw artwork using a copyrighted character label under Creative Commons. The character did not belong to the person posting the image therefore they have no rights to the character. Also be sure to check and make sure the user hasn't put restrictions on the use of their images.

Deviant art:

You can look in the Resources & Stock images section. Be forewarned not all the images are free for use, make sure you also check the user's rules. Many will not allow their images to be used outside of DA and most require you to contact them before using them.

Wikimedia Commons:

I haven't had much success with this site (I've found a grand total of 1 image), but there are lots of images available and best yet they're all free to use.

Stock Exchange:

I hate even mentioning this site because most the images I've ever found had rather strict usage restrictions.