This has been happening on the last 5 or six versions of XDK and is yet to be fixed.
Create an image button, set some styling (Not sure if that makes a difference) go to edit another page of the page containing the buttons and the images disappear from the buttons. The Tick disappears again the option and the selected image is shown as undefined. You have to re-tick the option and re-select the required button image. A real pain. Even the smallest edit like associating the button to a page causes this to happen.