Fancybox and Chrome / Firefox - image

Question

i got a "little" bug on my website. Here is the code im using:
product.tpl
{else}
href="{$link->getImageLink($product->link_rewrite, $imageIds, 'thickbox_default')|escape:'html':'UTF-8'}"
data-fancybox-group="other-views"
class="fancybox{if $image.id_image == $cover.id_image} shown{/if}"
global.css
.fancybox-skin {
background: #f4f5f7 !important; }
.fancybox-skin .fancybox-close {
width: 28px;
height: 28px;
background: none;
font-size: 28px;
line-height: 28px;
color: #333333;
text-align: center;
background: #f4f5f7!important;
-webkit-border-radius: 50px;
-moz-border-radius: 50px;
border-radius: 50px; }
.fancybox-skin .fancybox-close:hover {
color: #515151; }
.fancybox-skin .fancybox-close:after {
content: "\f057";
font-family: "FontAwesome"; }
Here is a Link to a product.
If you enlarge the picture by clicking on it, and hover to one of the sites and than jump to one of the other pictures you can see that the background is moving. In Internet Explorer it is working fine.
Now:
/* 1920x1200 ----------- */
#media only screen
and (min-width : 1681px) {
/* Styles */
body {
font-family: Arial, Helvetica, sans-serif;
font-size: 13px;
line-height: 1.42857;
background: #f4f5f7 url('http://evoxity.net/themes/default-bootstrap/img/background/1920x1200.jpg');
margin: 0 ;
background-repeat: no-repeat scroll;
-moz-background-size:cover;
overflow: auto;
background-position:center 0;
}
}