How to control the watermask show and hide?

http://www.wargame-club.com/3-day-molle-assault-backpack-black.html

After researched some time, I still don’t know which code controls the watermask show and hide. Now, I want to when click the thumbnail image which under the base image, there are four thumbnail image, not show the watermask on the base image, when the mouse hover on the base image, it shows the water mask.