The reason im wanting to do this is to update Letterboxd

Thanks for contributing new poster, only next time please also set correct language .

As for your question - The posters users contribute are available via our API to 3rd parties immediately after upload, so it is only on Letterboxd to update its DB to show it. I've removed the old poster (because it was against our guidelines), so your poster is now the only one. However in the case there are more posters for a movie, TMDB uses internal scoring algorithm to select default poster (based on language, poster quality and user votes) and your poster actually may not be selected.

Hey, thank you so much for your reply. I will add language next time. Im trying to add all of the NZ film commission official short films so ill be sure to do that too.

I checked the LB page, i think you taking down the old one did the trick.

Thank you again, and ill bear that in mind