The sites listed below are great sources for photos and other images. The subjects available will vary from site to site so be sure to check more than one if you can't find the image you are looking for.
IMPORTANT: Some of these sites allow photographers to place extra restrictions on how their photos are used. Be sure to check the usage rights for each individual image.
NOTE: Be sure to follow the instructions for downloading photos to get the best version of the image. If you just copy and paste the image that shows up in the search results list, you may end up with a poor quality version of the image.