It will be deleted as soon as an official poster is released.

Sorry but this makes no sense. It's either an official teaser artwork or it isn't (a deletion should not be postponed)! This just validates people uploading unofficial things like still frames to posters for upcoming or festival titles... and now book covers

Some users like to have a visual. I'm not a big fan of this practice, but Travis said they were fine a little while ago.

That's what the bible says:

Stripped down versions of official posters as well as posters made from promo pictures and logos are allowed. No posters made from material unrelated to the movie (e.g. book covers) or any unofficial material (e.g. candid pictures) unless there is no official poster. No "fan art" posters.