Hi,
Thanks for getting in touch. There’s no way of doing that with the admin at the moment I’m afraid. The reason we haven’t built it in yet is because the Instant Article version is, in theory, suppose to be pretty much the same as your Web version, and if a different Featured Image is set Facebook will flag it as being different and won’t allow it through the review stage.
If you’re comfortable editing code it would be fairly easy to use a custom field to set a new URL to a featured image jsut for the IA version. The filter you’d want in the plugin is wpna_facebook_post_get_the_featured_image (it returns an array, you’ll want to change the ‘url’ field).
I hope that helps.
All the best
