Adding my vote for this feature although apparently it's falling on deaf ears. Seems so basic. The most frustrating thing is even if you delete all other photos, the Evernote thumbnail still shows a photo which isn't even attached any more.
After reading the hundreds of comments with thousands of upvotes related to this issue. I sent a msg to their team via Help & Learning. I received a reply that my request would be forwarded to the team. Since this forum seems to be ignored maybe we should all send a request directly. Couldn't hurt right?