OK where to start... First of all, you have to get a banner or something. You can get these from the shops that membes put up.
Once you have the image, SAVE it to your computer.
Next, you can go to:
http://www.imageshack.us
Click on "Browse"
Choose the saved image you want to use.
Now click: "Host it!"
Left click on the adress there (The one on the top)
Now that it's highlighted, right-click and go to COPY
Now, go to the Edit Sig page and PASTE it on there.
And now you have a sig pictur!