It defaults to MSPaint, but you can change the definition of the program that's
       opened by changing (or adding) the following definition in REXEDIT.INI under the
       [Config] section

       BMP_EXE=<path/file to execute>

       For example,
       BMP_EXE=C:\Program Files\Accessories\Mspaint.exe

       You can change this to point to a different program, as long as the program
       supports loading the still image type you're using.


In the RexEdit.INI file, there is a reference to BMP_EXE:

BMP_EXE=C:\Program Files\Accessories\Mspaint.exe

Change the filepath to the program you want to run when you choose "Open paint soft"