Yeah, its easy cuz you do nothing but add it in. It uses opacity to create a rollover effect.

GLOBAL HEADER



<style type="text/css">
<!--
a img {
     filter:alpha(opacity=50);
     -moz-opacity: .5;
     opacity: .5
}
a:hover img {
     filter:alpha(opacity=100);
     -moz-opacity: 1;
     opacity: 1
}
-->
</style>


If you don't want sigs to have the rollover effect use this Insted


<style type="text/css">
<!--
a img {
     filter:alpha(opacity=50);
     -moz-opacity: .5;
     opacity: .5
}
a:hover img, .signature img, #signature img {
     filter:alpha(opacity=100);
     -moz-opacity: 1;
     opacity: 1
}
-->
</style>