Availability: gg
Once you have prepared your sandbox server for image creation you can do it with lc-gg-image-save tool:
$ lc-gg-image-save -i 123 -n mynewimage
Where 123 is an id of the sandbox server to be used as a base for new image and mynewimage is a name you want to give to your new image.
Note: at the time of writing preparing an image is not quite trivial and involves logging in to the box over ssh and running some scripts. Please refer to http://wiki.gogrid.com/wiki/index.php/MyGSIGoGrid wiki for details.