In my experience (on a Windows system), if I delete the picture that is selected as thumbnail, EverNote then pics the next picture as thumbnail. So I go through this process (delete, save note, undo) successively until it picks the picture I want. Tedious, but usually works.
Control Which Image Shown in a Note's Thumbnail/Snippet View
in General Feature Requests
Posted
In my experience (on a Windows system), if I delete the picture that is selected as thumbnail, EverNote then pics the next picture as thumbnail. So I go through this process (delete, save note, undo) successively until it picks the picture I want. Tedious, but usually works.