I found a a easy way to change the thumbnail in my iMac (sorry I don't use Evernote in Windows) I am using version 5.4.4 (402231) Choose the image you want use as a thumbnail within Evernote by highlighting it (it becomes light blue). Open with preview. Resize the image and save the file. In my experimenting I increased the image to150% of the original size. But it does not affect how it is displayed in Evernote. The thumbnail is immediately changed when the size fits the thumbnail algorithm described earlier in this thread. I only tried this once. But I am sure this will solve this once and for all.