I think I understand what you mean.
Load up the old machine the way you want the first image. Register it with the FOG server. Go into the FOG webui, create the image definition, go to the hosts and associate the host record with the image record. Perform and upload task.
Create a new image record for your second image. Setup the computer the way you want for the second image. Go into FOG and set the upload task so it creates the second image.
Create a new image definition in fog, change the host record to associate to the new image, setup the computer the way you want, upload the image.
One computer, multiple images.