Here is an extract from the README file:

 * Copyright (C) 1989-94 GROUPE BULL

                                XPM Version 3

WHAT IS XPM?
============

XPM (X PixMap) is a format for storing/retrieving X pixmaps to/from files.

Here is provided a library containing a set of four functions, similar to the
X bitmap functions as defined in the Xlib: XpmCreatePixmapFromData,
XpmCreateDataFromPixmap, XpmReadFileToPixmap and XpmWriteFileFromPixmap for
respectively including, storing, reading and writing this format, plus four
other: XpmCreateImageFromData, XpmCreateDataFromImage, XpmReadFileToImage and
XpmWriteFileFromImage for working with images instead of pixmaps.

WWW: http://www-sop.inria.fr/koala/lehors/xpm.html