If you have something that doesn't fit, it will fit it the best it can and leave space, all sites do that. A square image almost always works with most products. A small image, that's only like 2800px (and that looked expanded as its pixelated up close), doesn't fit on a duvet. You need a larger image.
And I was going to say what jessica said, clean crops, small signature, also watch for glare, the mountain scene may not print from the glare alone.
----Mike Savad