How can I make the picture or text I am dragging on top of a larger picture semi transparent

Imports System.Drawing.Imaging
Public Class Form1
 
    Private WithEvents PB As New PictureBox
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        PB.SizeMode = PictureBoxSizeMode.Normal
        PB.Dock = DockStyle.Fill
        Me.Controls.Add(PB)
    End Sub
 
    Private Sub Form1_Shown(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Shown
        SelectBackgroundImage()
    End Sub
 
    Private Sub SelectBackgroundImage()
        Using ofd As New OpenFileDialog
            With ofd
                .Title = "Select a BACKGROUND Image"
                .Filter = "Image Files(*.BMP;*.JPG;*.GIF)|*.BMP;*.JPG;*.GIF|All files (*.*)|*.*"
                If .ShowDialog = Windows.Forms.DialogResult.OK Then
                    Try
                        Using fs As New System.IO.FileStream(.FileName, IO.FileMode.Open)
                            PB.Image = Image.FromStream(fs)
                        End Using
                    Catch ex As Exception
                        MessageBox.Show("Image File" & .FileName, "Error Opening Image File", MessageBoxButtons.OK, MessageBoxIcon.Error)
                    End Try
                End If
            End With
        End Using
    End Sub
 
    Private Sub PB_DoubleClick(ByVal sender As Object, ByVal e As System.EventArgs) Handles PB.DoubleClick
        If Not IsNothing(PB.Image) Then
            Dim pt As Point = PB.PointToClient(Cursor.Position)
            Using ofd As New OpenFileDialog
                With ofd
                    .Title = "Select a smaller OVERLAY Image"
                    .Filter = "Image Files(*.BMP;*.JPG;*.GIF)|*.BMP;*.JPG;*.GIF|All files (*.*)|*.*"
                    If .ShowDialog = Windows.Forms.DialogResult.OK Then
                        Dim cm As New ColorMatrix
                        cm.Matrix33 = 0.5 ' values range from 0 --> 1
                        Dim IA As New ImageAttributes
                        IA.SetColorMatrix(cm)
 
                        Try
                            Dim overlay As New Bitmap(.FileName)
                            Dim bmp As New Bitmap(PB.Image)
                            Using G As Graphics = Graphics.FromImage(bmp)
                                G.DrawImage(overlay, New Rectangle(pt, overlay.Size), 0, 0, overlay.Width, overlay.Height, GraphicsUnit.Pixel, IA)
                            End Using
                            PB.Image = bmp
                        Catch ex As Exception
                            MessageBox.Show(ex.ToString, "Error Adding Overlay")
                        End Try
                    End If
                End With
            End Using
        Else
            MessageBox.Show("Please select a background image first...")
            SelectBackgroundImage()
        End If
    End Sub
 
End Class

                                              

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