I don't know which country you are from, but if you are an American you are legally liable for using images without permission even if you are completely unaware. It's absurd, really, since there is no way to tell who "owns" what.
If you want some completely free or low cost images, you could always try browsing Flickr and other image hosting sites. Some people offer images for sale, some license their images to be used by anyone for any purpose, and you could always message someone and ask about using their images or making some sort of deal. In addition to Flickr, there's Webshots, Picasaweb, Photobucket, and a whole bunch of others.
There are also some websites where you can get free images like icons and other graphics. I don't know any names off the top of my head. Perhaps others can chime in.