I had the same problem and I found out that it's because I don't have the image in the size that corresponds to the "View as" device selected in storyboard (bottom left).
If you've selected a device that needs the @2x image (example: iPhone 6s) and you only have the @3x image (as I did) you will see the question mark. If you add the size for the image that corresponds to the device size that you are using for storyboard, the image will appear in your imageView.
Basically, you want to have the image in all 3 sizes: @1x, @2x and @3x.
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