Entre o head:



<SCRIPT LANGUAGE="javascript"><!--
nereidFadeObjects = new Object();
nereidFadeTimers = new Object();
function nereidFade(object, destOp, rate, delta){
if(!document.all)
return
if (object != "[object]"){
setTimeout("nereidFade("+object+","+destOp+","+rate+","+delta+")",0);
return;
}
clearTimeout(nereidFadeTimers[object.sourceIndex]);
diff = destOp-object.filters.alpha.opacity;
direction = 1;
if (object.filters.alpha.opacity > destOp){
direction = -1;
}
delta=Math.min(direction*diff,delta);
object.filters.alpha.opacity+=direction*delta;
if (object.filters.alpha.opacity != destOp){
nereidFadeObjects[object.sourceIndex]=object;
nereidFadeTimers[object.sourceIndex]=setTimeout("nereidFade(nereidFadeObjects["+object.sourceIndex+"],"+destOp+","+rate+","+delta+")",rate);
}
}
if(document.images) {
var offimg = new Array()
offimg["cool"] = new Image(120,50)
offimg["cool"].src = "[INSERT]"
function imageOn(imgName) {
if (document.images) {
document.images[imgName].src = onimg[imgName].src
}
}
function imageOff(imgName) {
if (document.images) {
document.images[imgName].src = offimg[imgName].src
}
}
function setMsg(msg) {
window.status = msg
return true
}
function playIt() {
}
}
--></SCRIPT>


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No botão:


<img src="" width="" height="" align="middle" border="0"  style="filter:alpha(opacity=100)"
onmouseover="nereidFade(this,50,5,10)" onmouseout="nereidFade(this,100,50,5)">