3. This is a feature picture. (Adjective) OK
=> This picture is the feature (i.e., the main movie. Noun)
4. This is a featured picture.(Adjective) OK
=> This picture is featured. (i.e., shown as the main movie. Adjective)
5. I hope they keep him locked up forever. OK
=> He is locked up.
6. I hope they keep him lock up forever. (Not OK)
=> *He is lock up.