That's exactly how it's intended to be setup. Start with the image priority settings. Set which type you want displayed. Next, have only one image in the main type. This is the image that will be displayed. If the main image type does not exist then it looks for the second image type and the image within it... And so on.
There is no point in having multiple box front images, just have the one you want to see, all others are just taking up space. The only types of images you would want multiple of are gameplay and Fanart backgrounds, all other categories like cart front, 3D cart, 3D box, clear logo, box front/back/spine would all only need one per category, the rest is wasted space.