The website run by the U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies called 38 North, posted the imagery with tags showing how the conning tower of a new North Korean submarine can house 1-2 ballistic or cruise missile tubes.
The submarine was seen at the Sinpo South Shipyard in North Korea, which has seen significant infrastructural improvement recently.
Officials at the U.S. Korea Institute at SAIS speculated that a "shorter naval version of the Musudan intermediate-range ballistic missile, a Nodong medium-range ballistic missile, or naval versions of the solid-fuelled KN-02 short-range ballistic missile" could be the missile used aboard the submarine.
Obviously, a ballistic missile submarine would pose a new risk to South Korea. However, the analysts at Johns Hopkins pointed out that the imagery doesn't mean the North Koreans are necessarily close to completing the project.
Much like North Koreas ICBM program, the technology is still lacking north of the 38th parallel.