Create a calculation field with that case structure and put the calculation field on the layout. That will do for display. If you need to be able to drag and drop, make a small square of the actual container field colored black or gray or something similar and place it strategically close to the full size calculation field.
Ron Smith, MD, 'The Pediatric Guide For Parents'
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Forever And A Day For Laura Michelle
The results in your Case statement need to be the fields that contain the relevant images, not "file IDs" (whatever they are). Have a look at this thread, addressing a similar question recently: