Note Luke still has a glove on his hand. And since we didn’t see the mechanical hand fall off when he disappeared, all bets are off. 😉
I’ve seen it pointed out that the glove has been taken from a The Last Jedi photo and pasted into the new photo. So we can’t be sure what type of hand he will actually have in the movie.
Why would they do that? Just to misdirect?
Here is the TLJ photo:
https://media.vanityfair.com/photos/5924578a34765321e4a464ac/master/w_2000,c_limit/star-wars-portfolio-06-2017-ss01.jpg
Even the angle of his shoes and some of the creases in the robe are the same. So it’s possible that the “new” image is actually from the TLJ shoot but repurposed. Either they can’t or don’t want to show what he looks like now. It could just be because a new photoshoot with Mark couldn’t happen for some reason. Maybe this is what he will look like anyway, or maybe he will look different in some spoilery way they don’t want to show yet.