Multiple image buttons in form not working

I have two buttons in form. If they are button type, everything works fine. But when Ill change them to image button type, strange behavior occurs. When I press button 1, button 2 action is executed. If I press button 2, button 2 action is executed. That means that no matter which button I submit, action of the last one in form takes place.

I searched here, I googled but I didn't find any reference to this strange behavior. Have anybody come across this issue or better have a solution?

Yes, I know that I can use CSS, but even it that's ok, Ill like to know where the problem is.