The focus of that pic is not the Imperators on the back but the Castigator-class Titan in the center. And those are not Custodes but Imperial Fists. The whole pic is an hypothetical fan-take on the Imperium having recovered enough data on the destroyed Castigator Titan (from the Grey Knights novels) to actually make one (or more) in order to defend the Imperial Palace.
Never seen that pic at that high resolution tho. Nice catch.>>7708158
Keep seeing this fan-art. Is kind of nice to see the potential the Bling Marines can have given just how many resources/knowledge/technology they can have.