Well, all you gotta do is go to the signature option in your profile.
You'll then see a box to put text in.
If you wanna put a picture into your sig. then you have to locate the picture (say on the internet) and right click on the image.
Then go to "properties from that menu and copy the image's address (or URL as it's called).
Go back to your signature box and near the top you'll se an option to put in an image.
Click here and then it'll say to paste the image's URL in the text box. Paste the URL into this box and then it'll appear as the address in the signature text box.
Preview your signature and see if you like it! ^.^
If this doesn't help, I know there's a signature help thread somewhere on these forums.