You cant add a subview to a uiimageview, since drawRect: will never be called.
source:
"The "UIImageView class is optimized to draw its images to the display. UIImageView will not call the drawRect: method of a subclass. If your subclass needs custom drawing code, it is recommended you use UIView as the base class." from the apple docs:https://www.google.de/#bav=on.2,or.r_qf.&fp=90e2434f04e1ee9b&q=uiimageview+class+reference&safe=off
Solution:
As suggested, use a uiview to contain both the label and the imageview, and then bring the label to the top using -bringSubviewtoFront: of UIView. You can also use a UIViews backgroundimage to show your image and then have a label as a subclass in that view. Depends on the Situation you are in, i guess.
EDIT: YOU SHOULD READ THIS:
I misread the question, and it appears that you can add subviews to UIImageView, just like Zev pointed out in his comment. Right now im guessing that -bringSubviewToFront did the trick for you, and that the rest of my answer, while not really harmful, was unnecessary. Im sorry.