Sorry for the slow reply. I’m not sure I completely understand. The full screen image is on the front end? In the back end the location of the text is selected for each front end image? Where is this dropdown appearing? It seems to me it should be with the attachment or post attributes as part of their edit UI. If so, it should be a field within the existing form. The selected value is saved by hooking “save_post” or similar action.
When outputting the front end HTML, the classes to output that manage the text location would be determined by the value saved.
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