How can I find images to use in my thesis that are free/unrestricted by copyright?
This depends highly on your countries laws and regulations.

For example: The German copyright laws has some limitations for the use of works in the area of teaching and research.

With the CC licenses you risk less mistakes. But you have to take care of the correct distribution. 


name the author and source if the license contains BY.
don't change the file and only use it in the original composition when license contains ND (no derivate).
don't use media with licenses containing NC (not commercial) in a thesis or paper that contributes to (pending) patents. That could be interpreted to be a commercial use.
[changed:] according to the (IANAL-)comments one can use SA (share alike) licensed material without putting your thesis under the same license




In any case, pages that give information about the media-license are good sources, because you mostly have a clear licensing statement for any file. So you can easily decide whether and how to use it. I doubt that an author will change the license later on. You could use a web preservation repository service like WebCite to create evidence that the file had a specific license when you found and decided to use it.

Suggestions for search:


Wikimedia Commons
Flickr (thanks to Henning for the suggestion)