Convert a PPM image to an ILBM file. More information: https://netpbm.sourceforge.net/doc/ppmtoilbm.html.
ppmtoilbm path/to/file.ppm > path/to/file.ilbm
ppmtoilbm -maxplanes n -hamif|24if|dcif path/to/file.ppm > path/to/file.ilbm
ppmtoilbm -fixplanes n path/to/file.ppm > path/to/file.ilbm
ppmtoilbm -compress|nocompress|savemem path/to/file.ppm > path/to/file.ilbm