This playground project shows how to modify an image with CALayer (Swift 4)

CALayerImageExample.swift

//: # CALayer Example 2

import UIKit
import PlaygroundSupport

// view with white background color
var backgroundView = UIView(frame: CGRect(x: 0, y: 0, width: 400, height: 400))
backgroundView.backgroundColor = UIColor.white

PlaygroundPage.current.liveView = backgroundView

var imageView = UIView(frame: CGRect(x: 0, y: 0, width: 250, height: 250))
imageView.center = backgroundView.center
let myImage = UIImage(named: "nature.jpg")
backgroundView.addSubview(imageView)

imageView.layer.contents = myImage?.cgImage

imageView.layer.borderWidth = 4
imageView.layer.borderColor = UIColor.green.cgColor

imageView.layer.cornerRadius = imageView.bounds.height/2.0

imageView.layer.masksToBounds = true