I think most people don't want this. They'd rather have the most popular photo showing, whether it's their photo or not.
If you really want this, you could go over to Cubits.org and start your own database in your own cubit, the way Calif_Sue has done.
She has separate databases for separate categories. In each category, she lists her own plants and uploads her own photos to go with them.
http://cubits.org/Suehomegarde...
