When you lock or encrypt files on the android OS, apps outside of the locker or encrypter cannot access them, and while they are reckognized as valid media, they are also inaccessable as these apps are not capable of accessing or reading these files. As a response, these apps often say the file is corrupt, incompatible, or recommend to delete them. If you want the gallery app to display them properly again, you would need to unlock the files. This is a large down-side to privacy apps, as they are often times the only app that can access their contents.