======

有待改进，暂时先这样

<script type="text/javascript">
    var str =
'<div><img src="/uploads/allimg/100429/1_100429134118_1.jpg" /><br />'+
'<img  src="/uploads/allimg/100429/1_100429134118_2.jpg" /><br />'+
'<img src="/uploads/allimg/100429/1_100429134118_3.jpg" /></div>';
function getIMG(str) {
    var reg = new RegExp("<img[^>]* src=\"([^\"]*)\"[^>]*>","gi");
    var arrIMG = str.match(reg);
    var newIMG = [];
    for (var i in arrIMG) {
        var t = arrIMG[i].replace(reg, "$1");
        newIMG.push(t);
    }
    return newIMG;
}
var imgs = getIMG(str);
for (var i in imgs) {
    alert(imgs[i]);
}

======好的方法

function getIMG(str) {
    var reg = new RegExp("<img[^>]* src=\"([^\"]*)\"[^>]*>", "gi"),t,arrIMG=[];
    while (t = reg.exec(str)) {
        arrIMG.push(t[1]);
    }
    return arrIMG;
}

</script>