When Ensemble Studios, creators of Halo Wars, was closed a few projects went with it. Surprisingly one of those projects was another Halo title, though not much is known about it. These painting were from artist Dylan Cole and are listed as concept artwork for a cancelled title in the Halo Universe.
We do know Ensemble was working on a Halo MMO at one point, so most likely this beautiful cityscape was related to that. Still said to know the above shot will never get the full game treatment it deserves.
Source: Dylancolestudio.com