You could just use CSS hover. Javascript isn't really required:
#facebook{
width:100px;
height:100px;
background-image:url('image1.jpg');
}
#facebook:hover{
background-image:url('image2.jpg');
}
Your HTML:
<div id="facebook">Text here</div>
Working example: http://jsfiddle.net/NRFQ5/