I came over this Issue xcode 9 Could not load the image referenced from a nib in the bundle with identifier when I was playing around with Xcode 10.
No big deal, the solution is right in the linked topic, but unfortunately I don't have the 'Target Membership' Panel, when I select an Image.
Your images are inside a folder reference, indicated by the fact that the folder is blue. That means that only the folder can have target membership. The images inside it just come along for the ride.
(It also means that your images are not going to end up at the top level of the app bundle, which is why your nibs can't find them. You would need to refer to that image in the nib as Bilder/würfel3.png in order for it to be found. And if you wanted to load it in code, it would be even more complicated.)
