<script language="javascript">
    var oTime;
    function chgSize(args) {
        var oImg = document.all['oImg'];
        oImg.style.zoom = parseInt(oImg.style.zoom) + (args ? +1 : -1)  + '%';
        oTime = window.setTimeout('chgSize(' + args + ')', 100);
    }    
document.onmouseup = function() {
window.clearTimeout(oTime);
}  
</script>
<div style="height: 200px; overflow: auto;">
<img id="oImg" src="/Files/BeyondPic/2006-8/14/28youandi.jpg" style="zoom: 100%;">
</div>
<button onmousedown="chgSize(true);" onmouseup="window.clearTimeout(oTime);">↑</button>
<button onmousedown="chgSize(false);" onmouseup="window.clearTimeout(oTime);">↓</button>