帮忙看个小问题

<html>
<body>
<input type="button" onclick="a();" value="按钮">
<div id="b"><img id="img" src="http://i3.itc.cn/20101122/40c_7f5f269e_5ecf_4aff_900f_cb1baf2fbb89_0.jpg"></div>
</body>
</html>
<script>
function a(){
document.getElementById('b').innerHTML="<img src='http://i3.itc.cn/20101123/85c_05d76c65_e244_4255_b71f_6edfb2f3a61a_0.jpg' onmouseover='c('http://i1.itc.cn/20101122/241_a9963b9d_8d9b_4cb0_acd1_5944eb21056a_0.jpg');' />";
}
function c(img){
document.getElementById("img").src=img;
}
</script>
onmouseover出不来