I've been waiting for someone to reply to this, but i guess nobody knows.
Only info we have is "Pick a picture to be shown as a thumbnail. Make sure it has a 2000 x 1500 pixels resolution and reflects your topic. "
That's one big@ss thumbnail though and when you use that size, you need to crop it for said thumbnail(actually header) space
Had trouble submitting one article, support told me too many pics or pics are too big (filesize) but other articles i made with more and bigger pics got submitted np
While it still feels a bit buggy to add my pics in an article (shows broken image link often after uploading and sometimes only works after renaming the picture) i now tend to choose relative small 800/600 pics for the content of the article
I guess it's just an trial and error thing, you will learn along the way :/