Image buttons are 'old' type of buttonx, where you had to create three images, with specific names.
Since Shapes appeared with Captivate 6 I never used them anymore. Is there any reason why you are stuck with Image buttons? They had only 3 states, graphics ended on _up, _over, _down. Look in the Gallery, there is o image button that has also an InBuilt state correspoinding with Visited. I suspect the team has overlooked the fact that you cannot create an image button with 4 inbuilt states, because there is no 4th graphic available and that is the way Image buttons were designed many years ago. There is no way to switch graphics. Even a custom state will always look like the Normal state. But for a custom state, you can switch the type of button, go to a transparent button and change the color.
Sorry, didn't detect this before, because I never use them.
Shape buttons don't have that problem: you can fill the shape with any graphic, maybe you should switch to Shape buttons?