NSString *saveDirectory = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
    saveDirectory = [saveDirectory stringByAppendingPathComponent:@"/my_images"];
    NSFileManager *fileManger = [NSFileManager defaultManager];
    if( ![fileManger createDirectoryAtPath:saveDirectory withIntermediateDirectories:YES attributes:nil error:nil]){
        NSLog(@"create directory fail : %@", self.localDirectoryPath);
    }
    NSString *webImagePath =@"https://w...content-available-to-author-only...m.tw/images/srpr/logo11w.png";
    NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:webImagePath]];
    if( imageData != nil){
        NSLog(@"image data isn't nil");
    }
    NSString *localImagePath = [saveDirectory stringByAppendingPathComponent:@"image.png"];
    NSError *error = nil;
    if( [imageData writeToFile:localImagePath options:NSDataWritingAtomic error:&error]){
        NSLog(@"writeToFile: success: %@", localImagePath);
    } else{
        NSLog(@"writeTofILE: failure: %@ - %@", localImagePath, [error localizedDescription]);
    }