Tubemate Link
The developer (Devinco Mobile) distributes it exclusively via the official website: tubemate.net or tubemate.com (be cautious of fake sites).
Our partners at PhotoHound have several packs of free sky replacement images too.
PhotoHound is a place for photographers to discover how, where and when to capture the world's best photo spots. Signup is free and there's a great mobile app - check it out today!