Problem: I need a User's custom Image field to become their post's featured image
Solution: You should use the save_post action hook to automatically assign the custom user image field as post thumbnail. Please check how you can use save_post with the following doc: => https://toolset.com/documentation/customizing-sites-using-php/updating-types-fields-using-php/
You can find the proposed solution in this case with the following reply: => https://toolset.com/forums/topic/i-need-a-users-custom-image-field-to-become-their-posts-featured-image/#post-1967961
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