The code can only handle uncompressed format BMP files.

/**************************************************************************
 *  load_bmp                                                              *
 *    Loads a bitmap file into memory.                                    *
 **************************************************************************/


void load_bmp(char *file,BITMAP *b)
{
  FILE *fp;
  long index;
  word num_colors;
  int x;


  /* open the file */
  if ((fp = fopen(file,"rb")) == NULL)
  {
    printf("Error opening file %s.\n",file);
    exit(1);
  }


  /* check to see if it is a valid bitmap file */
  if (fgetc(fp)!='B' || fgetc(fp)!='M')
  {
    fclose(fp);
    printf("%s is not a bitmap file.\n",file);
    exit(1);
  }


  /* read in the width and height of the image, and the
     number of colors used; ignore the rest */
  fskip(fp,16);
  fread(&b->width, sizeof(word), 1, fp);
  fskip(fp,2);
  fread(&b->height,sizeof(word), 1, fp);
  fskip(fp,22);
  fread(&num_colors,sizeof(word), 1, fp);
  fskip(fp,6);


  /* assume we are working with an 8-bit file */
  if (num_colors==0) num_colors=256;




  /* try to allocate memory */
  if ((b->data = (byte *) malloc((word)(b->width*b->height))) == NULL)
  {
    fclose(fp);
    printf("Error allocating memory for file %s.\n",file);
    exit(1);
  }


  /* Ignore the palette information for now.
     See palette.c for code to read the palette info. */
  fskip(fp,num_colors*4);


  /* read the bitmap */
  for(index=(b->height-1)*b->width;index>=0;index-=b->width)
    for(x=0;xwidth;x++)
      b->data[(word)index+x]=(byte)fgetc(fp);


  fclose(fp);
}

from here