Look for this in your CSS:
CODE
.maintitle,
.maintitlecollapse{
background: transparent url(http://forums.invisionpower.com/style_images/1/tile_cat.gif);
border: 1px solid #FFF;
border-bottom: 1px solid #5176B5;
color: #FFF;
font-size: 12px;
font-weight: bold;
margin: 0;
padding: 8px;
}
(with a different URL for the background image of course). Change the URL to a pic of your own. You could also simply overwrite the tile_cat.gif image. Oh, and yes, you could use a full width image on a fixed width layout.