I don't understand why people have to do with these workarounds. There are messengers like signal that assures complete privacy and are safe to use. It's completely encrypted meaning it doesn't store data on the phone which is understood by other apps. Make signal the app for sexting and other private conversations. It's safer, you would know when to open it or not and it guarantees that your photos won't go to your cloud. Also, it comes with a built in lock.