According to NPR, the image is the result of a project by David Breashears, who has already climbed the highest mountain in the world five times. He used old images of Everest and its glaciers and combined them with new ones. The picture is estimate of 2 billion mega pixel.
His project aims to document the current state of glaciers in the Himalayas and how the mountain and those glaciers are being affected by climate change.
Source: The Washington post