By default, poster and backdrop changes apply platform-wide (as described above), but individual changes are possible in three scenarios: on your own review (or diary entry), on your own list, and on your profile favorites. Look for a checkbox labeled This item only when invoking the poster/backdrop selection dialog for these content types — if this option is checked, the customization only applies to that specific item. (For lists and favorites, customizing the backdrop on the first film in the list sets it for the list or profile.)
Resetting the poster for a film outside of the item in question will not affect any custom selections made in other reviews, diary entries or lists. To remove an individually customized poster or backdrop, return to the relevant content, open the selection dialog and use the Reset for item button, then Save for item. (The “This item only” option will be checked, indicating the poster or backdrop is customized specifically for this content.)