I tried resizing and it didn't work for me. And that would be a poor workaround anyway. It needs to simply be the user's choice.
I just started using Evernote (Premium) and too late, discovered that this photo thumbnail issue makes it somewhat useless for me. I want to add a photo of a sewing pattern cover (thumbnail), then a second photo of back of envelope with the details. Evernote is randomly selecting the details picture for the thumbnail. Pftt.