First of all, keep in mind that UIImagePickerController uses so much memory, an iphone 4 or 4s will give memory warnings if you do almost any conversion from UIImage to NSData. Having said this, you can solve your problem of replacing the shoot and retake buttons and have a better camera implementation by using AVFoundation. Here is a great example that is pretty straightforward:
and another:
http://www.musicalgeometry.com/?p=1297
Hope this helps, feel free to ask me more about it.