API page that documents this:

http://docs.openstack.org/api/openstack-compute/2/content/Create_Image-d1e4655.html (http://docs.openstack.org/api/opensta...)

I believe the image-create command also creates a snapshot as a side-effect of creating a new image.

In diablo, you can do this with create_image, which is how Horizon creates snapshots. With the python novaclient code, it looks like:

client.servers.create_image(instance_id, name)

where "name" is an arbitrary name you give the snapshot

Using the nova command-line client, it would be:

nova image-create [instance-id] [name]