Long story, short: I was trying to tag an image and it wasn't readily obvious that it was "Locked". I'm going to add a request to the Ticket Tracker (low priority, I'm sure) to make it more obvious that an image can't be edited, but wanted to make a forum post here mentioning it because my search for "locked image" came up with one, unrelated post, and searching for "locked" just returned all the (locked) forum posts (which wasn't useful).

I also think that it would be useful to mention that you can't edit/tag locked images in the (Un)Official Faq and the howto:tag wiki article (both of which I checked), but wasn't sure where I should put those two requests.