I have PhotoImact 11, but I believe the save dialog is the same in both 11 and 12.
To test I unchecked the save check box and clicked the OK box below.
It worked fine for me and remained unchecked the next time I rebooted and restarted PhotoImact.
It is important that you be sure to click the "OK" button after you uncheck the save box. If you click on the close X in the upper right hand corner instead, the change won't be registered. So be sure to click the "OK' button.
- Andy