Convert a PPM image to NCSA ICR format. More information: https://netpbm.sourceforge.net/doc/ppmtoicr.html.
ppmtoicr path/to/file.ppm > path/to/file.icr
ppmtoicr -windowname name path/to/file.ppm > path/to/file.icr
ppmtoicr -expand factor path/to/file.ppm > path/to/file.icr
ppmtoicr -display number path/to/file.ppm > path/to/file.icr