Example:

Put this code on your page:
<script> /* Gradual-Highlight Image Script II- By J. Mark Birenbaum (birenbau@ugrad.cs.ualberta.ca) Permission granted to Dynamicdrive.com to feature script in archive For full source to script, visit http://dynamicdrive.com */ nereidFadeObjects = new Object(); nereidFadeTimers = new Object(); /* object - image to be faded (actual object, not name); * destop - destination transparency level (ie 80, for mostly solid) * rate - time in milliseconds between trasparency changes (best under 100) * delta - amount of change each time (ie 5, for 5% change in transparency) */ function nereidFade(object, destOp, rate, delta){ if (!document.all) return if (object != "[object]"){ //do this so I can take a string too 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); } } </script>

Put this code for the image you want to have the opacity rollover on:
<img src="URLHERE.gif" style="filter:alpha(opacity=15)" onmouseover="nereidFade(this,100,70,20)" onmouseout="nereidFade(this,10,70,20)">