dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        NSData * imageData = [[NSData alloc] initWithContentsOfURL: [NSURL URLWithString: @"http://myurl/mypic.jpg"]];

        dispatch_async(dispatch_get_main_queue(), ^{
            weakSelf.imageView.image = [UIImage imageWithData:imageData];
        });

    });