This is in accordance with the Kim Family Regime playbook. Byungjin is the Kim Jong-un policy which incorporates the Military First Politics of his father and is essentially the simultaneous development of the north's nuclear program and the economy. Recall that the last revision of the DPRK constitution declared that it is a nuclear power. This is their logical progression to a "triad" capability (at least a photo shopped one as you noted). They want to be considered a nuclear power and in fact some speculate (as do I) that the only way they will return to the Six Party Talks is not for denuclearization (CVID - complete verifiable irreversible denuclearization) but for strategic arms limitation or reduction talks. They want to be treated as a nuclear power and to them it is a logical step to develop (or appear to develop) an SLBM.
28 October 2014
I should certainly hope our intelligence agencies were closely watching. But someone should also do an image search for submarine launched ballistic missiles and see if the photos below were taken from other launches. It would be fun to call out the Kim Family Regime if fake photos could be proven.