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 supports 2 and 4 bits per pixel, ppmtopict always generates an 8 bits per pixel file.
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.