I have downloaded some images of an adult nature onto my phone. My problem is, they show up as preview images whenever I try to compose a new tweet in the official Twitter app. I can't find any setting to turn this behaviour off.
Your best bet is probably to use an app like Keep Safe to hide "adult" photos/files. This keeps other apps from being able to see/interact with them, so they won't show up in the preview section of any apps.