The way you were doing it is actually working… unless the poster you’ve picked is no longer available (e.g. if it’s been removed at its source or there’s a licensing issue). Personally I prefer storing my posters as local assets alongside my media.
That being said, there’s already an existing suggestion to change/improce the poster locking behavior. If that’s what you have in mind, please comment/vote in that thread in order to help us avoid distracting or cannibalizing votes.
In addition there’s existing suggestions asking for a one-click option to lock all information of an item or an entire library respectively.
Unless I’m missing some unique aspect of your suggestion I’ll close this thread as a duplicate.
