Cannot subscript a value of type ‘[String : Any]’ with an index of type ‘UIImagePickerController.InfoKey’

The signature of the method has changed in Swift 4.2

func imagePickerController(_ picker: UIImagePickerController, 
  didFinishPickingMediaWithInfo info: [UIImagePickerController.InfoKey : Any])

and you have to write

guard let selectedImage = info[.originalImage] as? UIImage else {
    fatalError("Expected a dictionary containing an image, but was provided the following: \(info)")
}

You can figure out such terminology changes yourself by reading the documentation or by commenting out the entire method, retype the first few characters and use code completion.