tags are some what different from categories. Tags indicate a flat structure while an organization based on categories can be both flat and hierarchical. Keywords are used to imply important, descriptive and distinguishing words among other words in content. Since the content is visual Keywords loose their meaning. Lastly, Either way its just a matter of semantics pick the one that best expresses the purpose of the function to the common demographic of the user you are addressing. Do a small listening lab experiment or a usability test.