From UIImagePickerController Reference
Present the user interface by calling the presentModalViewController:animated: method of the currently active view controller, passing your configured image picker controller as the new view controller. On iPad, you can alternatively present the user interface using a popover as described in initWithContentViewController: and “Presenting and Dismissing the Popover” in UIPopoverController Class Reference.
UPDATE
From Why UIImagePickerController can not be push into navigation stack?
Apple does not allow stacking of navigation bars. Since the image picker has its own navigation bar, it cannot be placed in a navigation stack. The result would cause user confusion since there would be two bars, two sets of navigation items, two titles, etc.