u can view my working solution in jsfiddle: http://jsfiddle.net/avi_sagi/Ea78j/3/
CSS Rules
.thumb{
width:490px;
height:231px;
background:url('images/view_site.png') no-repeat center;
background-size:100%;
}
.thumb:hover .view_site{
width:490px;
height:231px;
background:url('images/white_house_website.PNG') no-repeat center;
background-size:128px 128px;
}
if your css is in external stylesheets remember to change the url of the background image or mention absolute url to be safe.
HTML
<div class="thumb">
<div class="view_site"></div>
</div>