In ExoGENI there are multiple ways that you can create custom Images. 

To get a list of currently available images on ExoGENI look here:
http://geni.renci.org:15080/registry/images.jsp

 * If you want to create an image of an OS that is not currently available in EG, or port your custom image to EG you can use VirtualBox to create and snapshot your image. 
   http://www.exogeni.net/2014/07/creating-a-custom-exogeni-image-using-virtual-box/

 * Modify an existing image loaded in a VM and take a snapshot
   http://www.exogeni.net/2014/06/creating-a-custom-image-from-an-existing-virtual-machine/
    
 * Use DropBox to store your ExoGENI images