Look first and foremost for Creative Commons images (you can google them) and see which are being offered up for use by photographers or illustrators around the world.
http://creativecommons.org/
Not all the Creative Commons licences are the same, so check out what you are actually allowed to do with the image you choose.
Flickr also has images which the community allow people to use, but many of those would be with a Creative Commons licence anyway. I have in the past used a couple of images from Flickr members, and found that just a short note requesting use of (and explaining how and where) an image - got me a pleasant reply. For individuals using them you will often get them for free... for companies that may be a different case though.
Or do what I did... learn how to use a camera and let others worry that they are not treading on your toes instead of the other way around!
ps. If you can find a way to use them legally you will sleep better at night!