The pictures, which were taken earlier this month, are said to show similar preparations by the country compared to those in August last year when it last tested a submarine-based missile.
Military experts have said that the hermit country, led by Kim Jong-un, is looking to upgrade is sea-based nuclear fleet and its submarine’s launch system Pukguksong-1.
The release of the images comes amid a period of heightened tensions between North Korea and the United States with both sides issuing threats and counter-threats.
US President Donald Trump said its “military solutions” were “locked and loaded” if North Korea acted “unwisely”.
Mr Trump’s comments came after North Korea’s state media issued a threat to the US that its military was “on standby to launch fire into its mainland, waiting for an order of final attack”.
It also added that Pyongyang was “capable of fighting any war the US wants”.
Joseph Bermudez, a specialist in North Korean defence and intelligence affairs, posted photographs on the 38 North blog of the US-Korea Institute at Johns Hopkins University.
He said: “Recent commercial satellite imagery reveals several developments suggesting that North Korea may be accelerating the development of the sea-based leg of its nuclear forces.
“Since the July report, netting or tarps have been suspended above both the fore and aft decks of the submarine obscuring any activity taking place beneath them.
“The only other time this was seen was during May-July 2016 and prior to the failed July 9, 2016 Pukguksong-1 test.”
Mr Bermudez pointed to activity on an experimental ballistic missile submarine at the Mayang-do navy shipyard and submarine base in Sinpo, which is on the east coast and where the country has previously carried out submarine-launched ballistic missiles.
He added: “The North may be preparing for a new series of ‘at sea’ test launches, has undertaken modifications or upgrades to the submarine’s launch systems, or is developing a more advanced version of the Pukguksong-1.”
Earlier this month US officials detected “highly unusual and unprecedented levels” of North Korean submarine activity.
Evidence of an “ejection test” had been found.
The Sinpo-class submarine is believed to have a range of around 1,200 miles.