The image (.dmg file) and associated files are saved on the server in the following location, where "x" is the volume number, and "image" is the image name you provided:
/Library/NetBoot/NetBootSPx/image.nbi
Important software license information
In accordance with the Network Install and Mac OS X Server software license agreements, you may use the software in this manner as long as you ensure that each end user of the image files is aware of and complies with the terms of the software license(s) that governs the use of the software contained within the image files. The Network Install and Mac OS X Server software may be used to install and reproduce materials so long as your use is limited to reproduction of non-copyrighted materials, materials in which you own the copyright, or materials you are authorized or legally permitted to reproduce.