So, the artist sent me a Resources.xcassets folder that I simply dragged into the project. I then jumped to interface builder, added a new UIImageView, assigned an image name from the xcassets, and to my surprise, I couldn't see the images. It shows as a blue question mark.
Notice, that the IB autocomplete is working, and when I run the application, the assets actually show up! But not in interface builder, which makes laying the assets around virtually impossible, especially with the size to fit not working...
Bug reported: 15733387