image placeholder on master slide that works as a button...?

Under the Actions dropdown list, bottom left of the editing screen in the Browser version of the forums, you could have found 'Edit'. But that is only available when your posting is the last in the thread. For other posts, only a moderator can edit if it is important.

No placeholders for shape buttons, which are the only interactive objects that can be added to master slides (or timed for the rest of the project. Shape buttons can be filled with an image. You could use that feature but have to explain how to change the used image. Be careful: Shape buttons have InBuilt Styles for Normal, Rollover and Down in Captivate 9 (it helps if you tell the version number), and the image would have to be changed in each of those states.