The location of an external program (that will accept .BMP files) that is called by 'File/ Run External Program' may be set here.
If the 'Edit Image with External program' checkbox is ticked, SPM saves a copy of the image at 'C:\windows\temp\' and names it 'TEMP.BMP'.
When the external program is run, 'C:\windows\temp\TEMP.BMP' is made a command-line argument.