this can be done by modifying the javascript.
As an example, for the full size grid gallery, go to the galleries/grid_fullsize/gallery.js file and find the following code:
//click
thumb.addEvent('click', function() {
this.showImage(arguments[0]);
return false;
}.bind(this, parts[0]));
and add this code after it:
//mouseover
thumb.addEvent('mouseover', function() {
this.showImage(arguments[0]);
return false;
}.bind(this, parts[0]));