Most posters are mainly shared for exposure, promotion.

However, downloading fan-art posters from DeviantArt and the like and then uploading is a no to me.

But, I'll quote lineker here:

If you are unsure about the laws where you live you should not add anything. If a production company would contact us and say that the promotional overview they have added to a press release is only allowed to be used by IMDb, that will will be a first. But maybe you are correct that IMDb owns the PDF-file shared to the press as well, but then that should be listed (and so far I have never seen any such attribution). It usually says for promotional purposes only with copyright belonging to the production company. Personally I prefer to rewrite the official stuff a bit, but it's a case by case decision.