Are they static images, i.e. they can only choose among 2 images that you never change? Or are you trying to get it so that they can upload a picture?
If it's static, have you thought about having both images "there" on the form, but using conditional formatting - hide them both if the dummy field is false. Then based on what button they press, update the dummy field (such as "showImageOne" or "showImageTwo") to true, and that will then show the right image.