You cannot currently register an image using the web interface, but you can use euca2ools from the command line to bundle, upload and register images which will then be available for use via the web interface:
https://www.eucalyptus.com/docs/eucalyptus/3.4/index.html#image-guide/img_add_existing.html
You can find an overview of image management tasks here:
https://www.eucalyptus.com/docs/eucalyptus/3.4/index.html#image-guide/img_task_intro.html