This option is only available when you are trying to share a Tweet to Fleets. Initially, it was spotted in development in May and released to iOS in April. To that moment it should be available to Android users as well.
How to test:
- Open any Tweet and tap Share.
- Select Share in a Fleet.
- Tap on the gallery or on the camera icon.
- Select or capture your custom background.
- 📲 Twitter is a popular social network build around short text messages.
- 📲 Twitter is also available in Beta and you can opt-in as a tester via Google Play.
- 📲 Twitter is also available in Alpha and you can opt-in by joining its Experiments Google Group
- 📲 Follow @TestingCatalog on Twitter for the most recent updates.