Image in imageView turned into a question mark in a Swift Xcode storyboard

Question

I'm studying Swift by following Stanford's lecture on iOS development. I'm currently adding an imageView to the storyboard. I have added an image to the imageView. However, I'm not sure what I have done, but suddenly the image turned into this:

I'm not asking for specific solution to the problem. I just want to know in general, what are some of the reasons that the image added could turn into a question mark.

Answer 1

My issue resolved when i just clicked and opened my assets catalog which contained my images

Answer 2

My issue is resolved when i provided all image set in "Assets.xcassets". Images for all required size (1x,2x,3x).