you can do it using the ribbon lighbox gallery type.
edit the gallery/ribbon_lightbox/js/galery.js file.
find the following code:
//click
thumb.addEvent('click', function() {
this.showImage(arguments[0]);
return false;
}.bind(this, parts[0]));
and change it to:
//click
thumb.addEvent('click', function() {
this.showImage(arguments[0]);
return false;
}.bind(this, parts[0]));
//hover
thumb.addEvent('mouseover', function() {
this.showImage(arguments[0]);
return false;
}.bind(this, parts[0]));