This is some sample code that mixes the contents of two pictures boxes together. It's not the most effecient, but it gets the job done quicker than any sample I could find online.
Option Explicit

Private Declare Function GetPixel Lib "gdi32" (ByVal hdc As Long, _
       ByVal X As Long, ByVal Y As Long) As Long
Private Declare Function SetPixel Lib "gdi32" (ByVal hdc As Long, _
       ByVal X As Long, ByVal Y As Long, ByVal crColor As Long) _
       As Long
Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" _
       (Destination As Any, Source As Any, ByVal Length As Long)

Private Type RGBAComponent
   Red As Byte
   Green As Byte
   Blue As Byte
   Alpha As Byte
End Type

Sub MixPictures(pctTarget As PictureBox, pctSource1 As PictureBox, _
   nSrcX1 As Long, nSrcY1 As Long, nWidth As Long, nHeight As Long, _
   pctSource2 As PictureBox, nSrcX2 As Long, nSrcY2 As Long, _
   nValue1 As Long)

   Dim nValue2 As Long
   nValue2 = 255 - nValue1

   Dim nCurColor1 As Long
   Dim nCurColor2 As Long
   Dim rgba1 As RGBAComponent
   Dim rgba2 As RGBAComponent

   Dim X As Long
   Dim Y As Long

   For Y = nSrcY1 To nSrcY1 + nHeight
       For X = nSrcX1 To nSrcX1 + nWidth
           nCurColor1 = GetPixel(pctSource1.hdc, X, Y)
           nCurColor2 = GetPixel(pctSource2.hdc, _
               X + nSrcX2 - nSrcX1, Y + nSrcY2 - nSrcY1)

           CopyMemory rgba1, nCurColor1, 4
           CopyMemory rgba2, nCurColor2, 4

           SetPixel pctTarget.hdc, X - nSrcX1, Y - nSrcY1, _
             RGB((rgba1.Red * nValue1 + rgba2.Red * nValue2) \ 255, _
             (rgba1.Green * nValue1 + rgba2.Green * nValue2) \ 255, _
             (rgba1.Blue * nValue1 + rgba2.Blue * nValue2) \ 255)
End Sub
Sample Usage:
    MixPictures Picture3, Picture1, 0, 0, Picture1.Width / _
        Screen.TwipsPerPixelX, Picture2.Height / _
        Screen.TwipsPerPixelY, Picture2, 0, 0, HScroll1.Value