Letterboxd favours original poster releases over DVD covers and artwork from subsequent releases (in most cases). We also favour versions of original posters with credits reduced or removed — such credits do not fare well at the small sizes we display posters at, and we feel in most cases the simpler visual presentation works better.
Unfortunately, due to the subjective nature of voting for posters, there isn’t an easy way to enable this to everyone’s satisfaction. But we’re happy for you to report the odd film using the flag icon in order to let us know if a superior poster exists on TMDb, notwithstanding the above caveats.marienbad commented
Having newly registered with Letterboxd, I feel the same about this as the other commentators in this thread. The "plastic and fake" quality of the poster currently displayed for "Das Cabinet des Dr Caligari" is particularly obvious – this American/English poster has nothing to do with the aesthetic of the original. At the same time, a gorgeous original poster is indeed featured on TMDb.