In order for your pop-up images to appear above your DHTML menu, you have to define the appropriate z-index to both elements: the menu and the element where the larger image appears into.
The z-index of the menu should be less than the z-index of the larger image. In order to change the z-index of the menu, change the property in the Main Menu Group Properties Editor (as shown in the image below).
You have to make sure that this is less than the z-index property of the larger image.

AllWebMenus z-index property