I guess the heading says it all - how do I upload a profile picture?

I'm going to assume you mean an avatar, since we don't have separate profile pictures (it will show on your profile page, too). Just go to this link and follow the instructions, it tells you about the maximum dimensions and filesize you can use: ucp.php?i=profile&mode=avatar

To find it again in future, just click User Control Panel in the menu above, then Profile and Edit Avatar.

Hope this helps.