Adding Image to Button


I have a form with a button on what am trying to do is set an image on the button when pressed.

I have set an image list on the button and not selected an image, when I click the button I tried to set the image index in two ways but it remains as -1 and not the 0 which is what I try and set it to below;

$butRun.ImageIndex = [System.Int32]0
$butRun.ImageIndex = 0

Any ideas or is there a bug?