Boxlet is a free, open-source server + mobile app system that allows you to take advantage of free space on a private cloud server as a backup/sync for your mobile photos. All you need is ssh and sudo access.
From the iOS app, you can specify any server:port where the Boxlet server is running. The server is highly configurable, including limitations on the amount of space to allow for backups. Assets can also be uploaded to Amazon S3 for virtually unlimited storage.
Please see http://boxlet.io or https://github.com/arktisklada/boxlet for more info