Están controladas por un javascript que maneja unos parámetros de CSS que hacen que la imagen se vuelva translucida.
Si ves el código fuente de la página, ves el script que lo maneja.
<script>
/*
Gradual-Highlight Image Script II-
By J. Mark Birenbaum (
[email protected])
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>
-------------------------
y luego en una imagen
<img border="0" src="pageimages/fan-amaro.gif" width="50" height="50" style="filter:alpha(opacity=30)" onmouseover="nereidFade(this,100,70,20)"
onmouseout="nereidFade(this,20,50,10)">
Donde los números de nereidFade indican los object, destOp, rate, delta indicados arriba, y en el comentario de los mismos te indica que es cada uno.
Este tipo de CSS (filters. Alpha. Opacity) es sólo válido para IE 5 y superiores. El NS no lo soporta.