Stuff like that gets answered quickest in Feedback and Help.
I can't remember for sure, but I think if it's a picture linked to somewhere else, you just insert it like any other picture. You can even open a second window with the "Post reply", use the buttons to get the coding to add a picture, copy and paste it into the window that has your sig stuff (in the User CP section). I assume you know how to at least put text in your sig since you've got stuff in there.