This program is part of Netpbm.
ppmtopict reads a PPM image as input and produces a Macintosh PICT file as output.
The generated file is only the data fork of a picture. You will need a program such as mcvert to generate a Macbinary or a BinHex file that contains the necessary information to identify the file as a PICT file to MacOS.
Even though PICT can have 2 and 4 bits per pixel, ppmtopict always generates an 8 bits per pixel file.
There are no command line options defined specifically for ppmtopict, but it recognizes the options common to all programs based on libnetpbm (See Common Options.)
The picture size field is correct only if the output is to a file since writing into this field requires seeking backwards on a file. However the PICT documentation seems to suggest that this field is not critical anyway since it is only the lower 16 bits of the picture size.