The xscanimage back end uses /root as its default save path. To change it to something like /tmp would require messing even more with its source code. The original xscanimage lets you select a save path, but that is hidden in xscanimage-peasy. Maybe I'll look at that again.Your peasyscan uses /root directory as the working directory.
As long as the program is terminated correctly, the temporary work files in /root will always be deleted.
I appreciate the confusion between "Quit" and "Done." "Done" is used to gracefully close the xscanimage window and send control back to the main window. "Quit" terminates the whole program. If you watch the flash video, you will see this in action.
Your screenie shows a "Save" button, but it is not needed by xscanimage. As soon as you click Scan, the raw pnm data is saved to /root/out.pnm.
Is your screenie just a mock-up or have you written the actual C code to generate it?
Thank you for the tip about viewnior displaying pnm files. However, if someone changed their default image viewer to another program, they might lose that ability and the preview function would be broken.