First determine what format your images are in. PBMPLUS will have converters for most image formats.
To convert an image to PostScript with the X Window System, you can display the image on the screen, and grab it with xv, which can save the image in a PostScript file.
A more general alternative in X Window System would be to use the PPM, PGM and PBM utilities in the X11R4 and X11R5 distributions.