When using multiple image_button elements in a form, there's no easy way to found out which one was clicked. This page contains a solution which allows you to identify the clicked image_button.
Recently I was building a module which showed a rather complex form. The form contained a mix of submit button and image_button elements.
In the submit handler I wrote some code to discover which button was clicked.
I made it work in the following case:
- I gave each button and image_button a unique attribute '#layout' of which the value uniquely identified the button.
- The image_button must not have a '#value' or '#default_value' attribute.
- In the submit handler, check the value of the $form_state['clicked_button']['#layout'] to find out which button was clicked!