Try setting the logging level to "Debug" (in the preferences). This might help identify what process is failing to run to completion.
The log files are automatically saved to ~/Library/Logs/System Image Utility/ and are time/date stamped. You should be able to view them using Console.app. The only reason it wouldn't get written out is if you force quit System Image Utility, instead of just hitting the stop button.
You can also view the log from within SIU from the View menu, or by hitting cmd-L.