The message purporting to be from the Nigerian extremist group was posted to Twitter pledging to follow the Caliph and to obey ISIS "in times of difficulty and prosperity."
The Nigerian Islamist militant group has traded grainy videos for slick productions. This week, Boko Haram posted a video purportedly showing the bodies of two beheaded men accused of spying.
The Islamist extremist group that has killed and kidnapped thousands in Nigeria is increasingly expanding its area of operation as it reportedly tries to establish an independent state.
The government has downplayed the severity of the attack, but Amnesty International, which released the images, says they prove the attack was on a "horrific scale."
The militants, who are waging a deadly war in Nigeria, had attacked a Cameroonian military camp, a government official said. Nigeria's neighbors are being drawn into the battle with the group.
Baga, in the country's northeastern Borno state, was seized a week ago. Amnesty International says that as many as 2,000 people may have been killed by the Islamist extremists in recent days.
The capture of the base and adjoining town is the latest attack by the extremist groups, which is also responsible for the mass kidnapping of school-aged girls in April.