revolution slider scroll down issue

Hi, when displaying the revolution slider located right after the menu in my homepage the layers scroll down the slider grid as I scroll down in the window browser showing the grey background. I do not understand why this is happening since I just changed the background-image in the home-default revolution slider from the theme. Can someone help me with this? What should I do to fix the problem?

My website is http://www.shopshipusa.com

thanks