void free_image ( struct image *img );
struct image *read_image ( char *filename );
void read_pgm_pixel_data ( struct image *img );
void read_ppm_pixel_data ( struct image *img );
void write_image ( char *filename, struct image *img );
void write_pgm_pixel_data ( struct image *img );
void write_ppm_pixel_data ( struct image *img );