From your view source:
<div id="bannerTwo" class="banners"><img src="images/logo.gif" alt="logo" title=" logo " width="468" height="60"
I suggest that you back out the banner that you have enabled first.
then place this image in this folder:
includes/templates/sophybluegr/images/logo.gif
and let zen Cart place the image in a normal way.
Then to center this edit the includes/templates/sophybluegr/css/stylesheet.css file and find this:
#logo {
float: left;
padding-top: 17px;
padding-left: 18px;
}
Change to this:
#logo {
margin: 0em;
text-align: center;
padding-top: 0px;
}
See if this gets you what you want