How to combine two UIImages of difference size without altering their original aspect ratios?

Question

My current method of combining UIImages come from this answer on SO, as well as this popular question on resizing UIImage with aspect ratio. My current issue is the following:<

Answer 1

Mackworth's answer in Swift 3.x

    let fullSize:CGSize = img.size
    let newSize:CGSize = fullSize
    let scale:CGFloat = newSize.height/fullSize.height
    let offset:CGFloat = (newSize.width - fullSize.width*scale)/2
    let offsetRect:CGRect = CGRect.init(x: offset, y: 0, width: newSize.width - offset*2, height: newSize.height)
    print(NSStringFromCGRect(offsetRect))     

    UIGraphicsBeginImageContext(newSize);
    self.pictureView.image.draw(in: offsetRect)
    self.annotateView.image.draw(in: CGRect.init(x: 0, y: 0, width: waterMarkImage.size.width, height: waterMarkImage.size.height))
    let combImage:UIImage = UIGraphicsGetImageFromCurrentImageContext()!
    UIGraphicsEndImageContext();
    return combImage;