Test Question (Hot spot)

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If you go to Preferences, and click the Form Elements tab, you can change the look of the buttons, including the option to add your own button.  Keep in mind that this applies to all radio buttons in the title. 

Another option is a bit more complicated, but gives you a lot of freedom.   And that is creating a question from scratch with an underlying question for the logic.  Put your image on a page and add whatever buttons or transparent shapes you want on top of it.  If you don't need "official" tracking, you can just add "on click" actions to the buttons or shapes to show a correct or incorrect message.  If you do need "official" tracking, then create a behind-the-scenes multiple-choice question that is not seen by the student.  On the buttons or transparent shapes you put on the image, make the "on click" action set the question variable to be correct or incorrect.