You should trigger your events within a handleButton press method. You don't preset arguments, buttonPress selectors pass one argument, the button that is sending the event.
[hangButton addTarget:self
action:@selector(handleButtonPress:)
forControlEvents:UIControlEventTouchUpInside];
- (void) handleButtonPress:(id)sender {
UIImage * imageToFaceDetect = // get your image
[self faceDetector:imageToFaceDetect];
}