Image rollover in Virtuemart products page

[outersquare]

Hey everyone... I'm hoping to add a rollover to the thumbnails on my Virtuemart category page? My site is: http://www.moderndayhippie.com.au/mdh_temp/index.php/products

I'm hoping that when the mouse rollovers the image, the image changes to another image (I'm wanting to darken the rollover image and add text - will be all graphics). For an example: http://www.larabar.com/products

Any suggestions on how I can achieve this? Many thanks in advance.