Well, either these leaks are legit and this guy is/knows the source of the Shadowlands banner art, or it's a hoax and this guy is/knows the source of the Shadowlands banner art.
The Riftlands art doesn't look very impressive and doesn't stand out to me as being from any particular Blizzard environment concept artist, but I suppose a lot of their environment concept art is just roughly blocked out to establish a mood.
The Journey's End piece is quite a bit more convincing.
He says he's got sixteen pieces, so I guess we'll see if he posts more. So far he's posted two in the span of about 40 minutes.