Yeah, you have to upload your logo somewhere. You can upload to a site like TinyPic without signing up:
(http://tinypic.com/)Or make an account with something like Photobucket & upload it there.
(http://photobucket.com/)(Of course there's a ton of other free image hosting sites too. Any are fine. Doesn't matter where you upload to.)
So after you upload your image, copy the "Direct link" (or equivalent)... that's your URL.