Interview Question for Software Developer at Google:

void ReverseImage (bool &image, int const &size)
{
  for (int i=0; i < size; i++) {
    for (int j=0; j < size; j++) {
       image[i][j] = !image[i][j];
    }
  }
}

void ReverseImage (bool &image, int const &size)
{
  for (int i=0; i < size; i++) {
    for (int j=0; j < size; j++) {
       image[i][j] = !image[i][j];
    }
  }
}

Surely the challenge is to flip the image not just to change white pixels black and black pixels white.

Assumptions: n is even; flip along the vertical axis

ReverseImage (image, length, width)
{
  center = width/2
  for (i=0 to length) {
    for ( j=1 to center) {
       if image[i][center-j] != image[i][center-1+j]{
         image[i][center-j] = !image[i][center-j]
         image[i][center-1+j] = !image[i][center-1+j]
       }
    }
  }
}

Technically length and width are both n but I thought the code is easier to read if we give them separate, meaningful names.

Instead of the if statement, you could always swap all the values
  temp = image[i][center-j]
  image[i][center-j] = image[i][center-1+j]
  image[i][center-1+j] = temp

But that is always three operations whereas the if statement is only 3 operations in the worst case.

why not use bitwise NOT operation?
For unsigned integers, the bitwise complement of a number is the "mirror reflection" of the number across the half-way point of the range of the unsigned integer type. For example, for 8-bit unsigned integers, ~x == 255 - x, which can be visualized on a graph, as a downward line, which effectively "flips" an increasing range from 0 to 255, to a decreasing range from 255 to 0. A simple use of this is that to invert the colors of a grayscale or RGB image where each color component for every pixel is stored as an unsigned integer, one simply needs to bitwise complement all these integers.