int XXX_desc(IPTR im); int XXX_load(IPTR im);
the description routine should read (or appears to read) from the
stream im->fp and fill three fields in the image structure,
i.e., the image width, height and image type
im->w, im->h, im->type
The load routine should fill the raster matrix.
In no case should the reading routine close the stream pointed
to by im->fp.