Cant change the state of an image on the master slide.

I'm trying to change the state of an object on the master slide using variables. 

On the master slide is a series of images which will change depending on when the response they give to a question. 

After they have got a question right or wrong, buy clicking the next button on the feedback slide it will trigger the variable. 

So currentlyI have it set that on clicking the next button on the correct feedback layer it will set a number variable to = 1.

If they click the next button the incorrect feedback layer it will set the number variable to = - 1. 

The image is then triggered to change on the master based on the value of the variable. 

I've stuck a reference in to check that the variable is adjusting and it is but the state on the image state is not changing.