Try doing it with code:
Objective-C:
theLabel.backgroundColor = [UIColor colorWithPatternImage:[UIImage imageNamed:@"blah"]];
Swift:
theLabel.backgroundColor = UIColor(patternImage: UIImage(named: "blah")!)
Or place an UIImageView behind the label (not recommended).
Update: placing an UIImageView behind a label was not recommended because then you would have to manage two views. But if you must do something that only an UIImageView can do this is still a good approach. I suspect that your app will actually run better this way.