sorry:my reg is not very good :(
<script>
var str="<img align=\"center\" alt=\"asdasdasd\" src=\"/image/2003/x.jpg\" height=\"20\" />\r\n<img height=\"20\" align=\"center\" src=\"/image/2003/y.jpg\"/>\r\n"
alert(str.replace(/<img.*src=\"([^\"]*)\"[^\/]*\/>/g,"<img src=\"$1\" alt=\"下载中…\"/>"))
</script>