+1 to adding the ability to choose custom thumbnail images.
What's the point of having visual cues for notes if they don't always properly represent the content you're trying to save? I understand that the "algorithm" may work for most people and that's great... but what would be even better would be a way to override with a custom thumbnail. Facebook lets you do this when you share links.
Some people work visually, some focus on text. But if the developers are trying to focus on useability and a visually expressive way to view notes (Card View), it would necessitate being able to have some control of the thumbnails provided.
Evernote is a great product... let's help make it even better!