Link using two background-images:

 a {
 width: 100px;
 height: 100px;
 display: block;
 }
 #a { background: url(pic1-normal.jpg) no-repeat; }
 #a:hover { background: url(pic1-hover.jpg) no-repeat; }

Link using only one background-image:

 a {
 width: 100px;
 height: 100px;
 display: block;
 }
 #b { background: #000 url(pic1.png) no-repeat; }
 #b:hover { background-color: #f00; }