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Код

    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private Image imgPNG;

        private void Form1_Load(object sender, EventArgs e)
        {
            // оригинал
            imgPNG = Image.FromFile("C:\\4-0.png");
            // выводим на форме
            pictureBox1.Image = imgPNG;
            // для обработки
            Bitmap bmp = new Bitmap("C:\\4-0.png");
            BitmapData bmpData = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height),
                    ImageLockMode.ReadWrite, PixelFormat.Format32bppArgb);
            unsafe {
                byte *pxData = (byte*)bmpData.Scan0;
                for (int j = 0; j < bmpData.Height; ++j) {
                    for (int i = 0; i < bmpData.Width; ++i) {
                        byte alpha = *(pxData + 3);
                        if (alpha > 0) {
                            *(pxData + 0) = 255;
                            *(pxData + 1) = 255;
                            *(pxData + 2) = 255;
                            *(pxData + 3) = alpha;
                        }
                        if (alpha == 0) {
                            *(pxData + 0) = 0;
                            *(pxData + 1) = 0;
                            *(pxData + 2) = 0;
                            *(pxData + 3) = 255;
                        }
                        pxData += 4;
                    }
                }
            }
            // unlock bits
            bmp.UnlockBits(bmpData);
            // saving file
            FileStream myFileOut = new FileStream("c:\\image.png", FileMode.Create);
            bmp.Save(myFileOut, ImageFormat.Png);
            результат обработки выводим на форму рядышком  оригиналом
            pictureBox2.Image = bmp;
            myFileOut.Close();
            
        }
    }