Sorry for coming in late!

I don't think that your C program is doing what you thing it should be.  Certainly not
what your list-based scheme program is.  Specifically, the array indexing in :

       for (j=0; j< size_y; j++) {
               for (i=0; i< size_x; i++) {
                       p.x = i;
                       p.y = j;
                       dist = distance_point_point(&p, &refp);
                       if ( dist > 255 ) {
                               block[i*j] = 255;
                       } else {
                               block[i*j] = dist;
                       }
               }
       }

should probably be block[j*size_x + i].  What you have there is
only setting a very limited subset of the elements of the array.

Cheers,

--
Andrew