[FR] Automatically pull image from another asset field if not otherwise specified

[terraindesign]

In most use cases, the social media image and an image already defined on the entry are one and the same (only with a different transform applied). It would be great to be able to specify in the SEO field settings that the default SEO image use an existing asset field.

I realize that this can be accomplished using craft.seo.custom on the template side, however, the client would not be able to preview this in the CMS — which would be extremely useful.