If it is a real piece of artwork made at Lucasfilm, and it was a work in progress poster for some potential scrapped ideas, I believe this poster will not be used as promotional material close to around December. This looks like something that could be put in The Art of Episode IX book when it comes out. Or, it could just be a really well put together fake poster that someone made.
No, it’s highly unlikely that this would appear in the Art of book. It’s not concept art, it’s using legit promotional photography of the actors in costume. For it to be fan made would be nearly impossible, for it to be a poster concept would be highly improbable. That it is an official poster is possible, and that it is a retail/licensed based artwork is most likely. The latter are often character collages like these, and usually when a leak emerges it is merchandise related. There will almost certainly be an official teaser poster at celebration but it will probably be a lot more minimal than this.
I do agree. It’s possible that this could be some promotional material for Star Wars Celebration this year, but the concept of C-3PO with Chewbacca’s bowcaster and bandolier doesn’t seem like an idea that Luacsfilm was make use of in any content these days. This is why it could be possible that someone made a poster themselves to troll fans online.
JJ does love giving other people Chewie’s bowcaster. It’s a comical image, but then again 3PO is a comical character. It’d be weird but I could see a scene where it happens. Doubt they’d put it on a poster, but that might just explain more why this is probably a merchandising thing - there’s going to be a 3PO + bowcaster toy.