imgvtopgm binary package in Ubuntu Focal riscv64

 This program can convert, compress, and decompress up to 4-bit grayscale
 images for displaying on the PalmPilot. It can take any pbm, pnm, pgm file
 generated by the netpbm package and convert it into a suitable image
 for the Pilot.
 .
 Together with netpbm many image formats, including JPEG, PNG, GIF, TIFF
 and BMP, could be converted into PDB format. This can be displayed on
 PalmPilot/III by viewers like "ImageViewer", "TinyViewer" or "Spec".

Source package