1，效果图如下：
（在图片左上角添加了一个半透明的logo图片）

2，为方便使用，我们通过扩展UIImage类来实现添加图片水印功能
（水印图片透明度，位置，边距都可以设置）

//--- UIImageExtension.swift ---
import UIKit
 
extension UIImage{
     
    //水印位置枚举
    enum WaterMarkCorner{
        case TopLeft
        case TopRight
        case BottomLeft
        case BottomRight
    }
     
    //添加图片水印方法
    func waterMarkedImage(waterMarkImage:UIImage, corner:WaterMarkCorner = .BottomRight,
                          margin:CGPoint = CGPoint(x: 20, y: 20), alpha:CGFloat = 1) -> UIImage{
         
        var markFrame = CGRect(x:0, y: 0, width:waterMarkImage.size.width,
                               height: waterMarkImage.size.height)
        let imageSize = self.size
         
        switch corner{
        case .TopLeft:
            markFrame.origin = margin
        case .TopRight:
            markFrame.origin = CGPoint(x: imageSize.width - waterMarkImage.size.width - margin.x,
                                       y: margin.y)
        case .BottomLeft:
            markFrame.origin = CGPoint(x: margin.x,
                                       y: imageSize.height - waterMarkImage.size.height - margin.y)
        case .BottomRight:
            markFrame.origin = CGPoint(x: imageSize.width - waterMarkImage.size.width - margin.x,
                                       y: imageSize.height - waterMarkImage.size.height - margin.y)
        }
         
        // 开始给图片添加图片
        UIGraphicsBeginImageContext(imageSize)
        self.draw(in: CGRect(x: 0, y:0, width: imageSize.width, height: imageSize.height))
        waterMarkImage.draw(in: markFrame, blendMode: .normal, alpha: alpha)
        let waterMarkedImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
         
        return waterMarkedImage!
    }
}

3，使用样例

import UIKit
 
class ViewController: UIViewController {
     
    @IBOutlet weak var imageView: UIImageView!
     
    override func viewDidLoad() {
        super.viewDidLoad()
         
        //给图片添加水印
        imageView.image = UIImage(named:"bg")?
            .waterMarkedImage(waterMarkImage: UIImage(named: "logo")!,
                              corner: .TopLeft,
                              margin: CGPoint(x: 20, y: 20), alpha:1)
    }
}