I'm trying to make the featured image required (without code, I hope), so member's can't create posts with no featured image (and make the archives pages look junky).
Secondly you aren't able to associate a custom field and make it the featured image per say. You theme will always attempt to pull the image from the featured image field.
A workaround to this is to create your custom post template and have it overwrite your theme's template and you can set in the custom template the custom field to be used as the main image of the post.