This isn’t concept art that was created prior to filming about what the characters might look like, so it’s possible that this is very much what the Marvel heroes will really look like in Avengers: Infinity War.
It also seems that this image shows us more than we’re supposed to see as Cas has since pulled a bunch of Marvel art from his Instagram page, including this image.
While Disney artist Steve Cas doesn’t go into details on his Instagram image about exactly what this art is for, he said the image is something he’s working on specifically for the upcoming movie.