I believe the duplication may be a bug. Harmless though.
It's always best to try to style the provided classes if possible (in your own self-added stylesheet), rather than adding more classes to the template.
That said, if you wanted to add your own style, carlwhat was nearly correct when proposing adding a 3rd parameter: it should have been the 4th parameter instead, and be only the name of the class you wanted to add.
eg:
Code:
echo zen_image_button(BUTTON_IMAGE_RETURN_TO_PROD_LIST, BUTTON_RETURN_TO_PROD_LIST_ALT, '', 'myNewClass');