HI Roberto,
including SGN into an existing Image is not supported at the moment... If I understand you correctly you have installed SGN on the machine (w/o the Client Config) and after that you have created the image - correct?
So please note that SGN creates the machine key upon the first reboot after the installation > the name of the current machine is taken. In case that you rename the machine after that the machine key will not be renamed!!! Probably this is the root cause for the "wrong" name of the machine key.
Anyway since the key also has a unique ID it should always be possible to determine the right key for each machine.
Again: A renaming of the key will not be possible > in case that you want the machine key match the machine name (which would be default when installing SGN after renaming a machine) you would have to completely uninstall SGN and reinstall it again.
Hope that helps =)
BR
Dan