Way to miss the point. That is precicely what makes him brilliant on so many levels.
a) Elba is an astonishing actor. He has the physical build for the role, and a truly remarkable voice.
b) Elba has an intimidating presense, which is great for Heimdall
c) The Gods in Marvel are aliens. They look human but they aren't. Heimdall's skintone is no more strange than his eyes, and can be seen to being linked to his powers.
d) The best reason. It brings the unthinking prejudices of the viewers out in public. People who are outraged or disturbed by a black viking god shoudl take a good look in the mirror and think about why this bothers them so much. Heimdall is practically a non-entity in the comics, but Idris Elba's character is great in the movies.
And this is, in part, why these pictures matter here as well. Scince Fiction matters. Good Sci-Fi is aware of the issues of the day, and deals with them. It's not just an easily digested burger.
Thanks for making me laugh out loud! With the above logic you should be able to rationalize Reese Witherspoon playing Rosa Parks.