/* The height is required for a special case, see below */
int height = surface->GetHeight();

/* Lock the image and get its buffer address */
BYTE* buffer = surface->LockBuffer();

/* If for some reason the operation failed, quit */
if(!buff)
    return 0;

/* Get the pitch - the size of an image row in bytes */
int pitch = surface->GetPitch();

/* Here's the special case: The pitch might be negative (I forgot when)
   in that case we have to do some funky math */
if(pitch < 0)
{
    pitch *= -1;
    buff -= pitch*(height-1);
}

/* We could now modify the buffer. Note that its upside down - the first row is the last one that is displayed. */

/* We're done, unlock the image */
surface->UnlockBuffer(buffer);