By calling dispach_async on the main queue, does that mean everything is run on the background thread?

NSURLSession *session = [NSURLSession sharedSession];
NSString *urlString = [[NSString alloc] initWithFormat:@"https://api.instagram.com/v1/tags/photobomb/media/recent?access_token=%@", self.accessToken];
NSURL *url = [[NSURL alloc] initWithString:urlString];
NSURLRequest *request = [[NSURLRequest alloc] initWithURL:url];
NSURLSessionDownloadTask *task = [session downloadTaskWithRequest:request completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {

    NSData *data = [[NSData alloc] initWithContentsOfURL:location];
    NSDictionary *responseDictionary = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:nil];

    self.photos = [responseDictionary valueForKeyPath:@"data"];

    dispatch_async(dispatch_get_main_queue(), ^{
        [self.collectionView reloadData];
    });
}];
[task resume];