- (void)downloadImageWithURL:(NSURL *)url completionBlock:(void (^)(BOOL succeeded, NSData *data))completionBlock
{
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
    [NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error)
     {
         if (!error)
         {
             completionBlock(YES, data);
         }
         else
         {
             completionBlock(NO, nil);
         }
     }];
}

UIImage *image = [UIImage imageWithData:serverData];
NSData *jpegData = UIImageJPEGRepresentation(image, 0.75f);

CGDataProviderRef imgProvider = CGDataProviderCreateWithCFData((CFDataRef) data);
CGImageRef imgRef = CGImageCreateWithJPEGDataProvider(imgProvider, NULL, true, kCGRenderingIntentDefault);

import Foundation
import ImageIO
import MobileCoreServices

func saveImageData(_ data: Data, to fileUrl: URL) {
    guard let provider = CGDataProvider(data: data as CFData) else {
        print("Could not create data provider from data")
        return
    }

    guard let cgImage = CGImage(jpegDataProviderSource: provider, decode: nil, shouldInterpolate: true, intent: CGColorRenderingIntent.defaultIntent) else {
        print("Could not create cgImage from provider")
        return
    }

    guard let imageDestination = CGImageDestinationCreateWithURL(fileUrl as CFURL, kUTTypeJPEG, 1, nil) else {
        print("Could not create an image destination from url")
        return
    }

    let options = [
        kCGImageDestinationLossyCompressionQuality: 0.75, // Compress quality (if required)
        kCGImagePropertyOrientation: CGImagePropertyOrientation.right.rawValue // Rotate image to the right (if required)
    ] as CFDictionary

    CGImageDestinationAddImage(imageDestination, cgImage, options)
    let imageIsWritten = CGImageDestinationFinalize(imageDestination)

    if !imageIsWritten {
        print("Image could not be written")
    }
}

let data: Data = …
let documentDirectoryUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
let fileUrl = documentDirectoryUrl.appendingPathComponent("image.jpeg")
saveImageData(data, to: fileUrl)