well, im a noob in php coding. but you can change the image and make it link to whatever you wish. to do that you need to edit in your template in the ACP.
Or you can try installing an ad hack to vbulletin.
does it need php coding ?
go to admincp >>Styles and templates > select the Fire strom >edit header ..
and replace the image with desired asvert..or image...
to find the image location right click on the image and copy image location..