Change Image Only If .src Matches?

I have a form with two fields: shape and color. where people select a shape an image of the shape appears, and a default color image appears.When they select a color, it changes the color image. But if they go back and change the shape, I don't want it to turn the color back into the default image. But I don't know how to do this script.Here's the function I have, but I don't know how to write it:

function changecolor(picName,imgName)
{
var idx = filepath.pic4
if (idx = "/images/leaf_shapes/entire/transparent1.gif")

[code]....