Your best bet is to use the Creative Commons search tool. Using this search tool, you can choose what search engine to use, depending on the type of material you are looking for:
Make sure that you select the checkboxes if you need items that can be used for commercial purposes and/or if you want to be able to modify the original. Once you click search, you will be taking to the search engine's results page.
Before reusing an image from these searches, it is always a good idea to double-check the permissions on the source page.
Once you find the image you want to use, examine the page where you found it. For example, take this photograph of the Babson Globe from Wikimedia:
You can see on the page of origin, the license and the definition of that license are listed below the image. The license says that you may share, remix, and share alike as long as you provide the proper attribution:
By Dpbsmith (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons
On some pages, it may be more difficult to find the licensing/permission information. Remember, just because you can't find this information, it doesn't mean that the image is not copyrighted! In this case it is better to ask permission than ask forgiveness - contact either the creator of the image (if you can find out who it is) or the owner of the website who may be able to point you in the right direction.