PNG/JPEG representation from CIImage always returns nil

Just begin a new graphics context and draw your grayscale image there. iOS 10 or later you can use UIGraphicsImageRenderer, for older iOS version syntax please check edit history:

Xcode 11 • Swift 5.1

func blackWhiteImage(image: UIImage, isOpaque: Bool = false) -> Data? {
    guard let ciImage = CIImage(image: image)?.applyingFilter("CIColorControls", parameters: [kCIInputSaturationKey: 0]) else { return nil }
    let format = image.imageRendererFormat
    format.opaque = isOpaque
    return UIGraphicsImageRenderer(size: image.size, format: format).image { _ in
        UIImage(ciImage: ciImage).draw(in: CGRect(origin: .zero, size: image.size))
    }.pngData()
}

You can also extend UIImage to return a grayscale image :

extension UIImage {
    var ciImage: CIImage? { CIImage(image: self) }
    func grayscale(isOpaque: Bool = false) -> UIImage? {
        guard let ciImage = ciImage?.applyingFilter("CIColorControls", parameters: [kCIInputSaturationKey: 0]) else { return nil }
        let format = imageRendererFormat
        format.opaque = isOpaque
        return UIGraphicsImageRenderer(size: size, format: format).image { _ in
             UIImage(ciImage: ciImage).draw(in: CGRect(origin: .zero, size: size))
        }
    }
}

---

let profilePicture = UIImage(data: try! Data(contentsOf: URL(string:"http://i.stack.imgur.com/Xs4RX.jpg")!))!

if let grayscale = profilePicture.grayscale(), let data = grayscale.pngData() {  // or Swift 4.1 or earlier -> let data = UIImagePNGRepresentation(grayscale)
    print(data.count)   //  689035
}