iTunes movies
I have tested this with purchased movies from the iTunes Store.
It is not possible to change the artwork for purchased movies across devices using iCloud. The same is true for The movie title, summary, etc.
Changes to movie titles will be saved locally, even if you remove the download and download it at a later point in time. Other devices won't receive changed movie titles.
Movies without DRM
Movies without DRM that you have added yourself to your iTunes Library can be synced locally to your devices. Changes made to the artwork of these movie files will be updated upon the next sync (via USB/Wi-Fi).