An “image” is a snapshot from which you can create new server instances.
From Rackspace’s own API documentation:
An image is a collection of files used to create or rebuild a server. Rackspace provides a number of pre-built OS images by default. You may also create custom images from cloud servers you have launched. These custom images are useful for backup purposes or for producing “gold” server images if you plan to deploy a particular server configuration frequently.