display two images on the screen together showing both with 50% opacity

I have 2 images (older and modern maps) and I want to show them as one on the screen. (I next start rotating and dragging corners until they match). My images are 7000x10000 pixels aprox.!!

I have the modifiable image in a picturebox, opacity .5. I tried to place the fixed image (as it is) as the form background image. But I only see the picturebox with the image at 50%. Without an image but with backcolor I see the image 50% and background color 50%. So the background shows through the picturebox. Setting both backcolor and backimage shows the color and not the image.

I then tried to use 2 pictureboxes, equal in size and location, each with an 50% opacity image. I only see the one that has "bringtofront".

I see in Paint.Net image editor when using layers, that it shows 2 images together, without much calculation. So DotNet can do it, but I don't know how. Any help please?

This is my code

    Dim imageData As New BldData     Public Sub CreateImg(fnd As String, fnm As String)        PiBxForgr.BringToFront()        PiBxBackgr.SendToBack()        Me.BackColor = Color.Red        PiBxBackgr.SizeMode = PictureBoxSizeMode.Zoom        Dim bckgrFileInfo = New FileInfo(fnm) ' on background        If File.Exists(bckgrFileInfo.FullName) Then            Dim bitMap = New Drawing.Bitmap(bckgrFileInfo.FullName)            Dim shownRect = New Rectangle(0, 0, bitMap.Width, bitMap.Height)            imageData.bitMapBackgr = bitMap.Clone(shownRect, bitMap.PixelFormat)        End If        Dim frgrFileInfo = New FileInfo(fnd) ' in foreground        If File.Exists(frgrFileInfo.FullName) Then            imageData.bitMap = New Drawing.Bitmap(frgrFileInfo.FullName)            imageData.shownRect = New Rectangle(0, 0, imageData.bitMap.Width, imageData.bitMap.Height)            rePaint()        End If        imageLoaded = True    End Sub    Private Sub rePaint()        PiBxForgr.SizeMode = PictureBoxSizeMode.Zoom        imageData.bitMapForgr = imageData.bitMap.Clone(imageData.shownRect, imageData.bitMap.PixelFormat)        PiBxBackgr.Image = SetImageOpacity(imageData.bitMapBackgr, 0.5)        PiBxForgr.Image = SetImageOpacity(imageData.bitMapForgr, 0.5)    End Sub    Private Function SetImageOpacity(image As Image, opacity As Double) As Image        Dim bmp As New Bitmap(image.Width, image.Height)        Using gfx As Graphics = Graphics.FromImage(bmp)            Dim matrix As New ColorMatrix()            matrix.Matrix33 = opacity ' from 0.0 to 1.0            Dim attributes As New ImageAttributes()            attributes.SetColorMatrix(matrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap)            gfx.DrawImage(image, New Rectangle(0, 0, bmp.Width, bmp.Height), 0, 0, image.Width, image.Height, GraphicsUnit.Pixel, attributes)        End Using        Return bmp    End FunctionClass BldData    Friend bitMap As Bitmap ' bitmap of picture in file, original, rotated or distorted    Friend bitMapForgr As Bitmap ' bitmap of picture on screen    Friend bitMapBackgr As Bitmap ' bitmap of picture on back    Friend shownMap As Rectangle ' part of bitmap shown    Friend scale As Integer ' total size / shown size    Friend shownmapOrgWidth As Integer    ... Properties ....End Class

and the images, modern, dated and program display