How can I flip an image inside a UICollectionViewCell? - Swift 4

Question

I'm working with Swift 4 trying to flip a card (that is a UIImageView) which is inside a UICollectionViewCell. I've two images and when I run the app, I can see one of them. But after the flip, what I get is a blue image instead of the second one.

Here's the code I've for the UICollectionViewCell class.

import UIKit

class CardViewCell: UICollectionViewCell {

    var cardFlipped = false
    var frontImage = UIImage(named: "image1")
    var backImage = UIImage(named: "image2")

    @IBOutlet var weak cardImage: UIImageView!
    @IBOutlet weak var flipCardButton: UIButton!

    @IBAction func flipCardAction(_ sender: Any) {
        flipCardAnimation()
    }

    func flipCardAnimation() {
        cardFlipped ? showFront() : showBack()
        contentView.reloadInputViews()
    }

    func showFront() {
        var image = cardImage.image
        image = frontImage
        flipCardButton.setImage(image, for: .normal)

        let transitionOptions = UIViewAnimationOptions.transitionFlipFromLeft
        UIView.transition(with: flipCardButton,
                          duration: 0.3,
                          options: transitionOptions,
                          animations: nil,
                          completion: nil)

        cardFlipped = true
    }

    func showBack() {
        var image = cardImage.image
        image = backImage
        flipCardButton.setImage(image, for: .normal)

        let transitionOptions = UIViewAnimationOptions.transitionFlipFromLeft
        UIView.transition(with: flipCardButton,
                          duration: 0.3,
                          options: transitionOptions,
                          animations: nil,
                          completion: nil)

        cardFlipped = false
    }
}

---

import UIKit

private let reuseIdentifier = "CardViewCell"

class GameViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource {

    var collectionData = ["image2", "image2", "image2", "image2", "image2", "image2", "image2", "image2", "image2", "image2", "image2", "image2", "image2", "image2", "image2", "image2"]

    @IBOutlet weak var collectionView: UICollectionView!

    override func viewDidLoad() {
        super.viewDidLoad()
        collectionView.delegate = self
        collectionView.dataSource = self

        // Set up the 4x4 grid
        let width = (view.frame.size.width - 60) / 4
        let layout = collectionView.collectionViewLayout as! UICollectionViewFlowLayout
        layout.itemSize = CGSize(width: width, height: width)
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        collectionView.reloadData()
    }

    // MARK: UICollectionViewDataSource

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return collectionData.count
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! CardViewCell

        cell.cardImage.image = UIImage(named: collectionData[indexPath.row])

        return cell
    }

    func collectionView(_ collectionView: UICollectionView, didDeselectItemAt indexPath: IndexPath) {

        let cell = collectionView.cellForItem(at: indexPath) as! CardViewCell

        let selectedImage = collectionData[indexPath.item]

        cell.flipCardAction(selectedImage)
    }
}