Creative Commons Image Search
I have been using Creative Commons (CC) licensed photos for some time now, finding them by using Flickr's advanced search option. Compfight is a website that makes that search easier and much quicker. Basically, all you have to do is to click beside "Creative Commons" to change the parameter to "Only," enter your search tag, and click "Search." Click on your chosen photo to open it in Flickr, where you can check the CC details, and then view sizes to choose to download one or to use the link to the size of your choice. You can then copy and paste the URL or CC code to complete the attribution requirement.
You can also further define the search by using Seek Original, which refers to the size of the image: "Only" will limit results to images saved in their original size, "On" will display images of all sizes, with an informational blue bar giving dimensions of those which are not original size, and "Off" will display images of all sizes without the blue bar. Your search results can also be filtered by using a Safe Search to display only images which are safe for work.
If you aren't familiar with the Creative Commons license, it is a license granting a set of baseline rights to the owner and requiring the work (image, video, audio, text) to be attributed to the original author. I won't go into details here, but basically, for a licensed work, you will see the CC logo along with a description of the guidelines for use. You are then given the opportunity and responsibility to use the work according to those guidelines. A CC license can be obtained for no cost from Creative Commons. When creating a license, the owner can decide if he or she wants others to have the right to use the work commercially and if others will be allowed to modify the work.