You should always ask permission before taking anyone's photograph and respect their decision if they decline. Photographs of North Korean officials or any non-tourist sites should be avoided. In addition to this you should seek advice from your guide as to whether it is acceptable to take photographs – if they say no, again, please respect this.
Please note that digital cameras are subject to being checked for content at customs prior to leaving North Korea.
Officially there is also a ban on taking camera lenses larger than 150mm in size – if you do bring lenses which exceed the limit you run the risk of them being confiscated upon arrival.