This can be caused by several things. To get actions working properly when using an image as button can be tricky. For sure the easiest way is using a transparant button with the actions showing / hiding the respective images on enter/exit. When you really insist on using the image as button ( i can think of scenarios where that would be preferred ) you can do that by using the change content action.
Add both images to the title. Hide the over one.
Add 2 actions to the image that needs to be the button
Action1: Mouse Enter -> Change Contents