An alternative value other than TRUE for Variable images

I am trying to make a volume control that follows the level of the volume graphically. I was hoping to swap out the png's based on a variable value. I tried placing another value other than "....+TRUE.png" in and Indigo acts as if anything other than NUL is TRUE. I was wondering if there is any other way of swapping images on a control page, other than toggling the True or False state of a variable image?

[jay (support)]

This wiki article describes how to do it. Don't forget to name the base image with a "+" at the end (imagename+.png).