When I first began in Amazon Video Direct, I had all kinds of problems getting the dimensions correct.
I did not know what they mean by 'art work', so I tried to get photos of my own art work of the course.
You can click onto the following link to see how ugly the thumbnail looks:
Click Here to view!
I've actually submitted news ones to them but still, they have not changed it yet.
So this course is going to save you from making the mistake I made.
Since then I've created much better looking thumbnails:
The following one is the 1200 X 1600:
Enjoy the course,