You can use the search box on the Photo Pin to find free photos. Note you just need a link back to the original source. The site is easy to use. Check it out. Here is an example:
When you find a piciture you want, download it in the size you want, copy the code to give credit, insert the picture where you need it, and then use the html button on your site so you can embed the code. The photo credit line will appear similar to how you see it on this blog post.
Try out Photo Pin and let us know what you think.