CSS中如何让鼠标经过图片时切换图片

<a id="bg" href="#">这里的文字不现实，只显示图片</a>

a{background:url(a.jpg) no-repeat left top;display:block;width:100px;height:100px;text-decoration:none;text-indent:-20000px}
a:hover{background:url(b.jpg) no-repeat left top;}

html: <div id="link"></div>

#link{ width:**px;
height:**px;
background-image:url(../picture1.jpg); }
#link a:hover{background-image:url(../picture2.jpg); }