It would be great to have a custom Asset Field that links to one of the uploaded photos. This would allow highlighting a specific visual clue to the asset when viewing the card. You can kind of do this already with a hacky workaround using the Embed Field but it would be nice to see 1st class support.
In the Photos section of the sidebar the photo could be marked as 'Featured'. If allowed in a list view, a thumbnail version could be displayed.