[cheat-master30]

You mean a favicon, right?

From W3.org:

<link rel="icon" 
      type="image/png" 
      href="http://example.com/myicon.png">

Replace type with the image type, and replace the example.com part with your domain, and the part after with the image address you want to use. You can use various types, .ico files being the most common, but PNG files and such being supported in some modern browsers.

You can also just place an image called favicon.ico in the directory of your site the forum is in (forum root directory) and browsers will often automatically request it. There is already the vBulletin favicon icon in your forum directory (main) by default, so the best way is just to overwrite it with your own image.