When photos are added to an entry you have to check mark what type of photo it is to complete the transaction (bloom, leaves, bark, entire plant, etc.).
Maybe what we can do is still use the thumbs up feature, I really like seeing what the public likes most, but as Sue said on rare occasions, at the discretion of the moderator, we would disable a certain photo from being the thumbnail for that plant?
I know I have slowly been going through the semp database and deleting my own photos of poor quality, or a struggling and pitiful semp. That was the only photo representatives of a particular cultivar. Now that we actually have good photos of those plants I can delete my bad one. : )