You could try this - setting the contenttop div and image widths in home_styles_nearly.css
change the rules in contenttop div to:
#outerWrapper #contentWrapper #contenttop {
height:218px;
margin:auto;
padding:0px 0px 0px 0px;
background-color: #666666;
text-align:center;
width:472px;
}
(You don't need to change the margins from px values to auto, but it makes it a lot easier to adjust the div width and you won't always have to adjust the margins at the same time!)
Then select the image and give that a width of 472px also.
I'm not sure how this plays out in IE, you may have to tweak the ie css too.
