CatImageStretch.swift

import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let separatorLine = UIView(frame: CGRect(x: 50.0, y: 0.0, width: 1.0, height: view.bounds.height))
        separatorLine.backgroundColor = .systemRed
        view.addSubview(separatorLine)
        
        // Cat photo can be found here: https://unsplash.com/photos/MEb2jandkbc
        let image = UIImage(named: "pinecat")!
        let imageView = UIImageView(image: image)
        let width = 200.0
        imageView.frame = CGRect(x: 50.0, y: 100.0, width: width, height: width * image.size.height / image.size.width)
        view.addSubview(imageView)

        imageView.layer.anchorPoint = CGPoint(x: 0.0, y: 0.5)
        
        let translateTransform = CGAffineTransform(translationX: -imageView.bounds.width / 2.0, y: 0.0)
        imageView.transform = translateTransform
        
        // Stretch image out
        UIView.animate(withDuration: 1.0, delay: 1.0) {
            let scaleTransform = CGAffineTransform(scaleX: 2.0, y: 1.0)
            imageView.transform = scaleTransform.concatenating(translateTransform)
        } completion: { didComplete in
            // Reset back to original state
            UIView.animate(withDuration: 1.0, delay: 1.0) {
                imageView.transform = .identity
                imageView.layer.anchorPoint = CGPoint(x: 0.5, y: 0.5)
            }
        }
    }
}

Example of the animation can be seen here.

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        let separatorLine = UIView(frame: CGRect(x: 50.0, y: 0.0, width: 1.0, height: view.bounds.height))
        separatorLine.backgroundColor = .systemRed
        view.addSubview(separatorLine)

        // Cat photo can be found here: https://unsplash.com/photos/MEb2jandkbc
        let image = UIImage(named: "pinecat")!
        let imageView = UIImageView(image: image)
        let width = 200.0
        imageView.frame = CGRect(x: 50.0, y: 100.0, width: width, height: width * image.size.height / image.size.width)
        view.addSubview(imageView)

        UIView.animate(withDuration: 1.0, delay: 1.0) {
            let scale = CGPoint(x: 2, y: 1)
            let newWidth = imageView.bounds.width * scale.x
            let scaleTransform = CGAffineTransform(scaleX: scale.x, y: scale.y)
            let translateTransform = CGAffineTransform(translationX: (newWidth - imageView.bounds.width) / 2, y: 0)
            imageView.transform = scaleTransform.concatenating(translateTransform)
        } completion: { didComplete in
            // Reset back to original state
            UIView.animate(withDuration: 1.0, delay: 1.0) {
                imageView.transform = .identity
            }
        }
    }
}

☝️ That's probably a better option doing it all with the transform and not messing with the anchor. But incase changing the anchor is needed for other reasons, here's how it can be done.

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let separatorLine = UIView(frame: CGRect(x: 50.0, y: 0.0, width: 1.0, height: view.bounds.height))
        separatorLine.backgroundColor = .systemRed
        view.addSubview(separatorLine)
        
        // Cat photo can be found here: https://unsplash.com/photos/MEb2jandkbc
        let image = UIImage(named: "pinecat")!
        let imageView = UIImageView(image: image)
        let width = 200.0
        imageView.frame = CGRect(x: 50.0, y: 100.0, width: width, height: width * image.size.height / image.size.width)
        view.addSubview(imageView)

        imageView.layer.anchorPoint = CGPoint(x: 0.0, y: 0.5)
        
        let translateTransform = CGAffineTransform(translationX: -imageView.bounds.width / 2.0, y: 0.0)
        imageView.transform = translateTransform
        
        // Stretch image out
        UIView.animate(withDuration: 1.0, delay: 1.0) {
            let scaleTransform = CGAffineTransform(scaleX: 2.0, y: 1.0)
            imageView.transform = scaleTransform.concatenating(translateTransform)
        } completion: { didComplete in
            // Reset back to original state
            UIView.animate(withDuration: 1.0, delay: 1.0) {
                imageView.transform = translateTransform
            } completion: { _ in
                imageView.layer.anchorPoint = CGPoint(x: 0.5, y: 0.5)
                imageView.transform = .identity
            }
        }
    }
}