Facebook has changed code and this has affected the size of the picture headers on all Facebook Groups, thus radically messing up people’s headers. For now, a new upload picture size of 820px by 384px will give a Group admin about the right size and proportions, but it’s still not ideal re: spacing and crispness. I’ll update this post when the new correct size is known.
Update: marketeers are suggesting 1920px as the ideal new Group cover picture size, for upload. That’s the new Kindle Fire HD 10″ size, so I’m guessing that’s perhaps why Facebook changed the size on Groups.