You might want to look at your registry
HKEY_CLASSES_ROOT\Application\mspaint.exe\shell\edit\command
This could be what your after but i've not tested it yet as i don't have any other paint software available. If you change the data section to link to your current imaging program it should work.
Note: always back-up your data (write down the original information so you can change it back if it don't work).
Hope this helps
Jeff