Ok, I found the problem. The image being displayed is company_logo.gif and is called from the altskin.css in the fashion_mosaic_gray/css folder.
Code:
#header .line1 .logo a:link,
#header .line1 .logo a:visited,
#header .line1 .logo a:hover,
#header .line1 .logo a:active
{
display: block;
width: 228px;
height: 50px;
background: transparent url(../images/fashion_mosaic/company_logo.gif) no-repeat left top;
}
Try changing it there.