What am I doing wrong here?

I'm trying to make a simple rollover that changes image from black/white to color. The images are displayed in b/w and all end in "_bw.jpg" and on mouseOver I want to change to color - thoses images all end in "_cr.jpg"

My javascript is:

Code:
function mouseOver(imgName)
{
//alert(imgName + "_cr.jpg");
document.imgName.src = imgName
...