To add an image button to master slide

No way. The only interactive object that can be used on master slides are the shape buttons, available since version 6, the most flexible interactive object because they can also be used on question slides, timed for the rest of the project. Since CP6 I never use image buttons anymore, you can fill any shape with an image, then turn it into a button:

Turn an Image into a Button - Captivate blog

Those are also the only buttons that can have a different label in the InBuilt states, look at this example:

Playing with Captivate 9 - Captivate blog