Unable to add product picture

First of all I want to thank you for the awesome theme! We love it!
Unfortunately I was unable to add product pictures to our products. In the editor-view the window in the sidebar to add a (“featured”) product picture doesn’t appear. Just the window for the product-picture-gallery is visible (the little pictures below). The visibility options at the top of the screen are already checked.
I hope that you can help us with that, it’s the last problem to solve before we can go out to the world.