Since this isn’t your photo and isn’t public, the only person you can add to it is yourself. Are you in the photo?

Flickr’s people tagging feature has interesting restrictions on who you can/can’t tag in other people’s photographs, depending on other privacy settings. The UI approach is smart, in that when you run into it you get a decent explanation. Although, I am starting to find that I no-longer have a firm grasp on what Flickr does and doesn’t permit me to do in different situations, not to mention whether (and where) this stuff is configurable.