If you're referring to the banner on your forum, that says vbulletin, then you need to go into your:
Admin CP > Style Manager > All Style Options > Common Templates > Header
You'll see a code in there that says this:
Code:
<img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" />
You'll have to replace that with something like this for your board, as this has our image in it:
Code:
<img src="http://legionofangels.net/banners/job960.jpg" border="0" alt="$vboptions[bbtitle]" />
If you're not referring to your vbulletin logo, then this obviously doesn't help you.