Is there a way to programmatically alter a field's schema? Is there a way to modify a fields schema programmatically? I'd like the image field to have a "Premium" checkbox, but I need to store this data somewhere. Also, it needs to work for current fields and future fields as they are created.
I believe you are looking for the API function hook_schema_alter.

Though you should also be aware that the Media module exists specifically to solve this particular problem in a much nicer way.