the code you have:
//click
thumb.addEvent('click', function() {
this.showImage(arguments[0]);
return false;
}.bind(this, parts[0]));
}.bind(this));
//mouseover
thumb.addEvent('mouseover', function() {
this.showImage(arguments[0]);
return false;
}.bind(this, parts[0]));
should be:
//click
thumb.addEvent('click', function() {
this.showImage(arguments[0]);
return false;
}.bind(this, parts[0]));
//mouseover
thumb.addEvent('mouseover', function() {
this.showImage(arguments[0]);
return false;
}.bind(this, parts[0]));
}.bind(this));