Reduce number of levels per pixel, creating a mirror image, C++

I have a greyscale image that has a resolution of 150 x 200 pixels.

Task:

Create a function to count the number of black pixels in the image and display this.

Create a function that will invert the image.

Create a function that will reduce the number of levels per pixel , that is - setting the lower 6 bits in each pixel to 0 using the bitwise AND operator. The function should reduce the number of levels used per pixel to four.

Create a function that will create a mirror image, that is - so that for every row, pixel value in element 0 should be swapped with pixel value in element 149, and element 1 with element 148 etc...

So far, I have completed the first two parts of the task, however I am struggling with creating correct working functions for the last two (shown by asterisk characters) , any ideas?

My code:

#include <QCoreApplication>
#include <iostream>
#include "ImageHandle.h"

using namespace std;


int CountBlackPixels (unsigned char PixelGrid[WIDTH][HEIGHT]);


void InvertImage (unsigned char PixelGrid[WIDTH][HEIGHT]);


void ReducePixelLevel (unsigned char PixelGrid[WIDTH][HEIGHT]);


void MirrorImage (unsigned char PixelGrid[WIDTH][HEIGHT]);


int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    unsigned char PixelGrid[WIDTH][HEIGHT];     // Image loaded from file


    // If the file "Parrot.png" cannot be loaded ...
    if (!loadImage(PixelGrid, "Parrot.png"))
    {
        // Display an error message
        cout << "Error loading file \"Parrot.png\"" << endl;
    }
    else
    {
        cout << "File \"Parrot.png\" opened successfully" << endl;

        // Demo of use of saveImage - to create a copy as "ParrotCopy.png"
        // This should be modified to save the new images as specified
        if (saveImage(PixelGrid, "ParrotCopy.png"))
        {
            cout << "File \"ParrotCopy.png\" saved successfully" << endl;
        }
        else
        {
            cout << "Could not save \"ParrotCopy.png\"" << endl;
        }
    }

    // Display number of black pixels ...
    cout << "\nNumber of black pixels : " << CountBlackPixels(PixelGrid) << endl;

    InvertImage(PixelGrid);

    {
        if (saveImage(PixelGrid, "ParrotInv.png"))
        {
            cout << "\nFile \"ParrotInv.png\" saved successfully" << endl;
        }
        else
        {
            cout << "\nCould not save \"ParrotInv.png\"" << endl;
        }

    }

    ReducePixelLevel(PixelGrid);

    {
        if (saveImage(PixelGrid, "Parrot4level.png"))
        {
            cout << "\nFile \"Parrot4level.png\" saved successfully" << endl;
        }
        else
        {
            cout << "\nCould not save \"Parrot4level.png\"" << endl;
        }

    }

    MirrorImage (PixelGrid);

    {
        if (saveImage(PixelGrid, "ParrotMirror.png"))
        {
            cout << "\nFile \"ParrotMirror.png\" saved successfully" << endl;
        }
        else
        {
            cout << "\nCould not save \"ParrotMirror.png\"" << endl;
        }

    }
    return a.exec();
}


int CountBlackPixels(unsigned char PixelGrid[WIDTH][HEIGHT])
{
    int row;
    int col;
    int count = 0;

    for (row = 0; row < WIDTH; row++)
    {
        for (col = 0; col < HEIGHT; col++)
        {
            if (PixelGrid[row][col] == 0)
                count++;
        }
    }
    return count;
}


void InvertImage (unsigned char PixelGrid[WIDTH][HEIGHT])
{
    int row;
    int col;

    for (row = 0; row < WIDTH; row++)
    {
        for (col = 0; col < HEIGHT; col++)
        {
            PixelGrid[row][col] = ~PixelGrid[row][col];
        }
    }
}


void ReducePixelLevel (unsigned char PixelGrid[WIDTH][HEIGHT])


{
    int row;
    int col;

    for (row = 0; row < WIDTH; row++)
    {
        for (col = 0; col < HEIGHT; col++)
        {
            *************************        
        }   
    }
}


void MirrorImage (unsigned char PixelGrid[WIDTH][HEIGHT])


{
    ***************************
}