Try adding the code below, then removing the img_forward and img_back hover functions.
var overview = $('.ngg-galleryoverview'),
// vp = viewport width
vp = overview.width(),
// edgeMargin = margin to add on either side of the thumbs
edgeMargin = 20,
// maxw = width of all thumbs together side-by-side
maxw = $('.gallery_thumbs').css('padding','0 ' + edgeMargin + 'px').outerWidth();
overview.bind('mousemove', function(e){
overview.scrollLeft( (e.pageX - overview.offset().left) * (maxw - vp) / vp );
});
Update: Added comments to the code to prevent someone in the future from cursing my name trying to figure out what is going on.