To create your own OS image, you'll have to build it and upload it into Glance. We use an open source tool called veewee to build our images. We have several veewee definitions on github - https://github.com/dreamhost/veewee-definitions
Since we're using Ceph for storage of images, you can't use the QCOW format. Images created will need to be stored in RAW format.
We don't have any documentation for creating your own images yet, but it's something we'll be working on.