具体算法请参看《vb.net 教程 5-13 图像处理之像素处理 6》

黑白：

    '黑白1
    'http://blog.csdn.net/uruseibest
    Private Sub btn2Color1_Click(sender As Object,e As EventArgs) Handles btn2Color1.Click
        Dim destImg As New Bitmap(sourceImg.Width,sourceImg.Height)
        Dim sourceData As BitmapData = sourceImg.LockBits(New Rectangle(0,sourceImg.Width,sourceImg.Height),ImageLockMode.ReadOnly,PixelFormat.Format24bppRgb)
        Dim destData As BitmapData = destImg.LockBits(New Rectangle(0,ImageLockMode.WriteOnly,PixelFormat.Format24bppRgb)

        Dim pSource As IntPtr = sourceData.Scan0
        Dim allBytes As Integer = sourceData.Stride * sourceData.Height
        Dim rgbvalues() As Byte
        ReDim rgbvalues(allBytes - 1)
        Marshal.Copy(pSource,rgbvalues,allBytes)

        Dim pos As Integer = 0
        Dim R,G,B As Integer
        Dim avgValue As Integer

        For j As Integer = 0 To sourceData.Height - 1
            For i As Integer = 0 To sourceData.Width - 1
                B = rgbvalues(pos)
                G = rgbvalues(pos + 1)
                R = rgbvalues(pos + 2)
                avgValue = (B + G + R) / 3
                If avgValue >= 128 Then avgValue = 255 Else avgValue = 0
                rgbvalues(pos) = avgValue
                rgbvalues(pos + 1) = avgValue
                rgbvalues(pos + 2) = avgValue

                pos = pos + 3
            Next
            pos = pos + sourceData.Stride - sourceData.Width * 3
        Next

        Dim pDest As IntPtr = destData.Scan0
        Marshal.Copy(rgbvalues,pDest,allBytes)

        sourceImg.UnlockBits(sourceData)
        destImg.UnlockBits(destData)

        picDest.Image = destImg
    End Sub

    '黑白2
    'http://blog.csdn.net/uruseibest
    Private Sub btn2Color2_Click(sender As Object,e As EventArgs) Handles btn2Color2.Click
        Dim destImg As New Bitmap(sourceImg.Width,B As Integer

        Dim HistGram(255) As Integer

        For j As Integer = 0 To sourceData.Height - 1
            For i As Integer = 0 To sourceData.Width - 1
                R = rgbvalues(pos + 2)
                HistGram(R) += 1
                pos = pos + 3
            Next
            pos = pos + sourceData.Stride - sourceData.Width * 3
        Next

        Dim threshold As Integer

        Dim allSum,allCount As Integer
        For k As Integer = 0 To 255
            allCount += HistGram(k)
            allSum += k * HistGram(k)
        Next
        threshold = allSum / allCount

        pos = 0
        For j As Integer = 0 To sourceData.Height - 1
            For i As Integer = 0 To sourceData.Width - 1
                R = rgbvalues(pos + 2)
                If R >= threshold Then R = 255 Else R = 0

                rgbvalues(pos) = R
                rgbvalues(pos + 1) = R
                rgbvalues(pos + 2) = R

                pos = pos + 3
            Next
            pos = pos + sourceData.Stride - sourceData.Width * 3
        Next


        Dim pDest As IntPtr = destData.Scan0
        Marshal.Copy(rgbvalues,allBytes)

        sourceImg.UnlockBits(sourceData)
        destImg.UnlockBits(destData)

        picDest.Image = destImg
    End Sub