With Win-7 using Ghost walker you may not be able to change the SID using the Ghost walker
In such scenarios it is recommended to use sysprep manually a easy method for creating image with sysprep
What is sysprep?
The System Preparation tool (Sysprep) is a technology that you can use with other deployment tools to install Microsoft Windows operating systems with minimal intervention by an administrator or technician. Sysprep is typically used during large-scale rollouts when it would be too slow and costly to have administrators or technicians interactively install the operating system on individual computers.
To know more about it please visit the following site: http://technet.microsoft.com/en-us/library/cc783215%28WS.10%29.aspx
How Sysprep Works: http://technet.microsoft.com/en-us/library/dd744512%28WS.10%29.aspx
Instruction to run the sysprep manually: -To run sysprep goto run C:\windows\system32\sysprep. -Double click on the sysprep.exe -Put a check on the generalize and choose shutdown -Reboot the client machine with boot cd -Once the machine booted into the Ghost proceed with Image creation
NOTE: After you run the sysprep manually it will remove all the machine specific info so you can use it as master image.