Image copyright KCNA/Uriminzokkiri Image caption The officer appeared to be overcome with emotion as he climbed onto his leader's back

North Korea's test of a rocket engine last weekend was accompanied by the usual state media propaganda - but one image of its leader celebrating stood out in particular. What is the likely explanation?

The engine test was claimed to be a success, a "new birth" for North Korea's rocket industry. Kim Jong-un was certainly happy.

In pictures released by state news agency KCNA, he was seen watching the missile from afar; grinning in a control centre; shaking hands with jubilant officers - then, giving an elderly man a piggyback.

Who would leap onto the back of a dictator such as this, and why?

Observers say the mysterious man is not a known figure in North Korean politics. He is thought to have played a key role in the engine test, and most likely interacted with Mr Kim previously.

North Korean observer Michael Madden says his uniform's insignias indicate he is a mid-level officer of the KPA Strategic Force, which is in charge of missile forces used for offensive attacks.

While the image was almost certainly stage-managed, "it wasn't completely machinated or fabricated", says Mr Madden, who is with the US-Korea Institute at Johns Hopkins University.

"It was more a signal of allowance and encouragement than something completely machinated by an image maker." North Korean propaganda films have in the past shown citizens being allowed to approach Mr Kim.

'Friendly and convivial'

The main purpose of the picture would be to burnish Mr Kim's domestic image as a jovial man of the people.

While Mr Kim tries to project a "recalcitrant and uncompromising" image internationally, at home "it is a different story", notes Professor Jae-Cheon Lim, of the Korea University in Seoul.

"We know he is very strict with elites when they don't obey his orders. But in general towards the people his propaganda image is friendly and convivial."

Image copyright KCNA/Uriminzokkiri Image caption In another photo released by KCNA, the same officer was seen embracing Kim

It stands in stark contrast to his predecessors, who sought to be feared more than loved. "Nobody would dare piggyback his father or even his grandfather," says Mr Madden.

"But this fits into the image [Kim] Jong-un has tried to cultivate - that he is more open, on an interpersonal basis, than his father.

"It conveys a certain sense of political confidence in his rule and leadership of the country. If he didn't feel secure, then he wouldn't allow these images to be disseminated - he would need to appear distant and cold."

Image copyright KCNA/Uriminzokkiri Image caption Mr Kim was also photographed joking and laughing at the engine test site

The image also suggests that Mr Kim is, for now, in good health.

He was spotted limping and using a cane in 2014, leading to speculation that he had gout, and limping again as recently as late 2016.

Borrowing from football's playbook

Piggybacking after a win may be more commonly seen on football pitches rather than in North Korean propaganda pictures, but Mr Kim is known for taking a sports management approach to his weapons development programmes.

"When a test is conducted, civilian and military personnel [are told they] should regard it as a sports competition - they win some, and lose some," says Mr Madden.

"They won't 'win' or meet technical specifications all the time, and when they 'lose' they study their performance and what happened."

Image copyright KCNA/Uriminzokkiri

But for all its contrived spontaneity, it does not mean that Mr Kim is not genuinely happy in the photo.

Prof Lim points out that he had good reason to celebrate, with an apparently successful rocket engine test putting him one step closer towards his nuclear goals - and sealing his legacy.

"In the propaganda annals, his grandfather was the liberator of Korea through its anti-Japanese guerrilla war. His father succeeded in maintaining the regime even under economic poverty.

"But Kim Jong-un became leader quickly and has no significant achievements to point to so far.

"If North Korea becomes a nuclear state, it becomes his achievement."