A colleague, Eugene Eric Kim, has increasingly been uploading freely licensed photos to Wikimedia Commons. Excellent!
The more you get involved with Commons, the more you’ll probably want to move files into categories; and if you’re working with more than one or two files at a time, you’ll probably get tired of manually entering category information.
An excellent tool for file categorization is Cat-a-lot. So I made this short video, to demonstrate how to get started with it. (The video below is embedded from YouTube; if you prefer to view the video on Commons, here’s the link.)