Array of UIImages

As you can see from the error Xcode/Playgrounds is providing, the problem here is that your function getElements() is expecting to return a UIImage, yet you are returning a String. Additionally, the initialiser UIImage(named:) is expecting a string, but you're passing in an image.

This can be fixed in one of two ways:

1 - Rewrite the getElements() function actually return a UIImage. Of course this will likely have to be an optional, as randomElement() returns an optional element to handle cases where the array is empty.

func getElements() -> UIImage? {
    let elements = ["rectangle", "circle", "triangle"]
    guard let randomElement = elements.randomElement() else {
        return nil
    }
    return UIImage(named: randomElement)
}

And then at the point of usage, you can just do something like:

cell.contents = getElements()?.cgImage

2 - Alternatively, you could update your function getElements() to return a string. This is basically the same, but it moves the logic to test the optional to outside of the getElements() function.

func getElements() -> String? {
    let elements = ["rectangle", "circle", "triangle"]
    return elements.randomElement()
}

And then at the point of usage, you have to test the optional, and use it to initialise the image:

if let element = getElements() {
    cell.contents = UIImage(named: element)?.cgImage
}

Personally, I prefer option one, as it removes the burden of checking the nil string from the point of usage. But it all depends on what you're trying to achieve. I would also perhaps consider renaming the function to something like randomImage(), as getElements() is perhaps a little unclear, but that's just personal preference.

Hope that helps.