Calling CSS - Display An Image Based On A Value

I am trying to use Javascript to display an image based on a value. I have the image positions in my .css. Just to make it easy if my var x=0 i want to call the "position 1" div, or else i want to call the "position 2" div. How can i make this possible. I have positions because i have a background image and i want the different pictures to be in a different location on the background based on the value. Below is my code that
layout.css

#layout {
position: relative;
top:0%;
z-inderx:-1;
}
#date {
position: absolute;
top:5%;
left: 50%;
[Code]....