Banner resizing question
 

Hey I recently started working on forums. I'm new vbulletin and have some questions. My site I'm working on is: http://abyssaddicts.com/forums/index.php

if you click the link above you can see the site and the current banner. Now I know that the banner is too large. I was wondering when it's made smaller how I could get the banner to stay the width of the forums and resize with the forums as someone changes the size of the window the site is viewed in. Any help would be greatly appreciated.