Tried to upload a picture right now. After clicking on the button for file selection, I get a "you must be logged in to do that" kind of message and the log in screen. Obviously, I was previously logged.
Are you able to reproduce this issue? Which Firefox version and what OS?
Also, the site logs you out automatically after some time (2 weeks, I think). Is it possible this happened between when you loaded the page, and when you tried to actually upload pics?