div {
    width: 161px;
    height: 149px;
    background: url(http://3.bp.blogspot.com/_HGPPifzMEZU/Rw4ujF12G3I/AAAAAAAAAWI/bc1ppSb6eKA/s320/estrelas_09.gif) no-repeat center center;
    background-size: 50% 50%;
    transition: background-size 2s ease-in;
    -moz-transition: background-size 2s ease-in;
    -web-kit-transition: background-size 2s ease-in
}
div:hover {
    background-size: 100% 100%
}

<div>
hey
</div>

http://jsfiddle.net/ubcka/14/