ViewController.swift

private func loadThumbnailImage() {
    let size = imageView.bounds.size
    let scale = traitCollection.displayScale
    let url = Bundle.main.url(forResource: "Wallpaper", withExtension: "jpg")!
    DispatchQueue.global(qos: .userInitiated).async {
        let image = UIImage(thumbnailOfURL: url, size: size, scale: scale)!
        DispatchQueue.main.async { [weak self] in
            self?.imageView.image = image
        }
    }
}