well, nothing like a good mystery/conspiracy, even with a Disney film!!!
but yeah. it is very weird. there are TONS of fan-art based on everything on the internet. if those images are just fan-art, the sites shouldn't have to remove them. they'd just need to update the page with Disney's statement and alert readers that the images are not official and are fan-made.
usually, studios ask sites to remove something that's leaked ( meaning it's real ). like, say, bootlegged trailers that are recorded at a convention, for example. and even there, there are tons of fan-made movie trailers out there. they don't get pulled. it's the ones that contain the actual leaked content which the studio pulls.
so, my guess is that those images are "real." maybe not the posters themselves, but the character models are. they're probably early renders, using existing character models ( Rapunzel for Anna ) to get a rough idea of what the new character would look like in 3d ( Anna's outfit DOES match the one in Disney's official concept art they released a few months ago ).
somehow they got leaked, and then they were released as is or the models were photoshopped over the backgrounds to create the posters.