如何让一张图片颜色由浅到深变化

<SCRIPT language=javascript1.2>
<!--
function high(which2){
theobject=which2
highlighting=setInterval("highlightit(theobject)",50)
}
function low(which2){
clearInterval(highlighting)
which2.filters.alpha.opacity=40
}
function highlightit(cur2){
if (cur2.filters.alpha.opacity<100)
cur2.filters.alpha.opacity+=10
else if (window.highlighting)
clearInterval(highlighting)
}
</script>
<img onmouseout=low(this) onmouseover=high(this) style="FILTER: alpha(opacity=40)"src="/logo.gif" >