[PinkMilk]

If trying to "stretch":

Admincp > Styles & Templates > Style Manager

header template find:

<img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" />

Replace with:

<img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" id="logo" />

Scroll down to near bottom and add to "Additional CSS Definitions" box:

img#logo {
width:400px;
height:200px;
}

Obviously change to desired width and height.

Note: As mentioned this will only stretch the image and may/probably will distort it, it would be better to redo the logo in photoshop (or alternative software).