rollover effect on product

Hello.
I have a problem, may be that you can help me.
http://www.l2gshop.com/
for product image:
with css I applied a rollover effect that displays the image in black and white and hover displays the image in color.
But on mouse always displays the first image in the gallery and not the featured image.
Where can I inervenire to make sure that always show the featured image?
regards
Francesco