No forum would support the posting of html code because you could potentially use it to haXX0r everyone and everything XD
If you were trying to get your portable ID... example
<a href="http://us.playstation.com/playstation/psn/visit/profiles/TwilightLynk"><img src="http://fp.profiles.us.playstation.com/playstation/psn/pid/TwilightLynk.png" width="230" height="155" border="0" /></a><br/><a href="http://www.us.playstation.com/psn/signup">Get your Portable ID!</a>
Then you should do it like this...
[ url=http://us.playstation.com/playstation/psn/visit/profiles/TwilightLynk ][ img ]http://fp.profiles.us.playstation.com/playstation/psn/pid/TwilightLynk.png[ /img ][ /url ]
(without the spaces, obviously)
(though the maximum height for the forum sig images are 150px so it'll be automatically resized...)