xpmtoppm
Convert an X11 pixmap to a PPM image.
TLDR
Convert an XPM image to a PPM image
$ xpmtoppm [path/to/input_file.xpm] > [path/to/output_file.ppm]
Store the transparency mask of the input image in the specified file
$ xpmtoppm --alphaout [path/to/alpha_file.pbm] [path/to/input_file.xpm] > [path/to/output_file.ppm]