gistfile1.m

      WPSImageDownloaderCompletionBlock completion = ^(UIImage *image, NSError *error) {
         if (image) {
            NSData *pngData = UIImagePNGRepresentation(image);
            [[self imageCache] cacheData:pngData forKey:cacheKey cacheLocation:WPSCacheLocationFileSystem];
            cell.imageView.image = image;
         } else {
            ZAssert(error, @"Image download error: %@\n%@", [error localizedDescription], [error userInfo]);
         }
      };

      cell.imageView.image = [self placeholderImage];
      WPSImageDownloader *downloader = [[WPSImageDownloader alloc] init];
      [downloader setRetryCount:5];
      [downloader setCache:[self imageCache]];
      [downloader downloadImageAtURL:imageURL completion:completion];