Наложение нескольких изображений. Сохранение полученного изображения

Public Class Form8
    Private p1, p2, p3, p4, bsImage, rzImage As Bitmap
    Private Sub Form8_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
        bsImage = Image.FromFile("baseImg.png")
        p1 = Image.FromFile("p1.png")
        p2 = Image.FromFile("p2.png")
        p3 = Image.FromFile("p3.png")
        p4 = Image.FromFile("p4.png")
        Update_Click(Button1, Nothing)
    End Sub
    Private Sub Update_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        rzImage = New Bitmap(PictureBox1.Width, PictureBox1.Height)
        Dim gr As Graphics = Graphics.FromImage(rzImage)
        gr.DrawImage(bsImage, 0, 0, bsImage.Width, bsImage.Height)
        If CheckBox1.Checked Then gr.DrawImage(p1, CInt(TextBox1.Text), CInt(TextBox2.Text), p1.Width, p1.Height)
        If CheckBox2.Checked Then gr.DrawImage(p2, CInt(TextBox4.Text), CInt(TextBox3.Text), p2.Width, p2.Height)
        If CheckBox3.Checked Then gr.DrawImage(p3, CInt(TextBox6.Text), CInt(TextBox5.Text), p3.Width, p3.Height)
        If CheckBox4.Checked Then gr.DrawImage(p4, CInt(TextBox8.Text), CInt(TextBox7.Text), p4.Width, p4.Height)
        PictureBox1.Invalidate()
    End Sub
    Private Sub Save_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click
        rzImage.Save("Rezalt.png", System.Drawing.Imaging.ImageFormat.Png)
        MsgBox("Файл сохранен!")
    End Sub
    Private Sub PictureBox1_Paint(sender As System.Object, e As System.Windows.Forms.PaintEventArgs) Handles PictureBox1.Paint
        Dim g As Graphics = e.Graphics
        g.DrawImage(rzImage, 0, 0)
    End Sub
End Class