I dare say this is already hidden somewhere in the bowels of Tech Help or FAQs forums:
How to add a url info:
http://absolutewrite.com/forums/misc.php?do=bbcode#url
...and, bonus:
How to add an image info:
http://absolutewrite.com/forums/misc.php?do=bbcode#imgcode
Note that you can make your sigline pic "active" by putting it inside the url tags, so if folks click on the pic they get taken to your website, e.g.
[ url="http://yourwebsite.com"][ SIGPIC][/SIGPIC][/url]
(Remove the spaces before the [ brackets, which I had to insert to stop the board software from executing the tags.)
Edit: and I just found out by accident -- you can insert text between the sigpic tags that will display whenever anyone hovers their mouse over the pic, e.g.
[ url="http://yourwebsite.com"][ SIGPIC]Come visit my website![/SIGPIC][/url]
-Derek