The silhouetted figures facing seawards are straining to pick up a cheaper mobile signal from neighbouring Somalia, hoping to establish a tenuous link with relatives abroad.
"So many pictures of migrants show them as bedraggled and pathetic ... but this photo is not so much romantic, as dignified," said jury member Susan Linfield.
Djibouti is a common stop-off point for migrants heading from nearby countries like Somalia, Ethiopia and Eritrea in search of a better life in Europe and the Middle East.
"It opens up discussions about technology, globalisation, migration, poverty, desperation, alienation, humanity," said jury member Jillian Edelstein of the photo, which was commissioned by National Geographic magazine.
Reuters photographer Goran Tomasevic, from Serbia, won first prize in the spot news stories category for a dramatic narrative series from Syria depicting a rebel attack on a government checkpoint.
France's Phillipe Lopez of Agence France-Presse won the spot news singles category with a photograph of typhoon survivors in Tolosa, the Philippines, carrying religious iconography in front of a field of rubble.
Getty's Brent Stirton, a South African, topped the category for single staged portraits with a picture of five blind albino boys from West Bengal, India. Dressed in matching pink shirts and blue trousers, they appear to gaze stiffly at the camera.