It really is officially known now as The Dark Knight Trilogy, as per the DVD/Blu-ray release this month, so I think it should be changed to that and the poster accordingly.

why not "Batman: The Dark Knight Trilogy"

For now, series and groups of movies on TMDb are known as collections. We might be changing this at some point to match more closely the marketing titles of such groups of movies but this is our policy right now.

ok so then can we go back to "Batman: The Dark Knight Collection "?

I think its better to leave it as "Collection" anyways. Now days they turn a trilogy into 4 or 5 films years later.

There's a set of people who want the collection title renamed to The Dark Knight Trilogy; another group that wants the name Batman juxtaposed to the collection title, and so on. It does seem to be commonly referred to as variations of [Nolan's] Dark Knight trilogy/series in film reviews and marketing, so I renamed the collection to the existing title. It's going to be impossible to please everyone, and for some reason people are very particular about this collection.