VB.NET paint application - vb.net

Question

I'm facing two problems in my application:
The Undo Function
The Drawing Part
When i draw on the picturebox , it draws very well, when - let's say I want to undo an action, it undo's it but when I click back on the picturebox it reacts like a redo function ,all the drawings appear back on the image.
the second problem is : i want to be able to edit a picture so i load a image by clicking on a listview item but due to something i'm missing the image it is not show but instead it shows a white background in which i am able to draw.
bellow is the code i am using
Imports Microsoft.VisualBasic.PowerPacks
Public Class Form1
Public drawgraph, g As Graphics
Private redoBuffer As New Stack(Of Image)()
Private undoBuffer As New Stack(Of Image)()
Dim color As Color
Dim UndoStack As New Stack(Of Bitmap)()
Dim xStart, yStart, xEnd, yEnd As Integer
Public Drawbitmap As Bitmap
Dim Drawgraphics As Graphics
Dim myPen As New Pen(color.Black, 4)
Dim myColor As Color = color.Black
Dim myPenWidth As Integer
Dim myBGColor As Color = color.White
Dim Drawing As Boolean
Private Sub drawMyline()
PictureBox4.Image = Drawbitmap
Drawgraphics.SmoothingMode = Drawing2D.SmoothingMode.HighQuality
Drawgraphics.DrawLine(myPen, xStart, yStart, xEnd, yEnd)
End Sub
Private Sub PushUndo(ByVal b As Bitmap)
UndoStack.Push(b)
End Sub
Private Function PopUndo() As Bitmap
If UndoStack.Count = 0 Then
Return Nothing
Exit Function
End If
If UndoStack.Count > 0 Then
Return UndoStack.Pop
End If
End Function
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Drawbitmap = New Bitmap(PictureBox4.Width, PictureBox4.Height)
Drawgraphics = Graphics.FromImage(Drawbitmap)
PictureBox4.Image = Drawbitmap
Drawgraphics.Clear(color.White)
myPenWidth = NumericUpDown1.Value
xStart = -1
yStart = -1
Drawing = False
End Sub
Private Sub PictureBox7_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles PictureBox7.Click
Dim bmp As Bitmap
bmp = PopUndo()
If bmp IsNot Nothing Then
PictureBox4.Image = bmp
End If
End Sub
Private Sub PictureBox4_MouseDown(ByVal sender As System.Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox4.MouseDown
Drawing = True
PushUndo(PictureBox4.Image.Clone)
End Sub
Private Sub PictureBox4_MouseUp(ByVal sender As System.Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox4.MouseUp
Drawing = False
End Sub
Private Sub PictureBox4_MouseMove(ByVal sender As System.Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles PictureBox4.MouseMove
If Drawing Then
xStart = e.X
yStart = e.Y
drawMyline()
End If
xEnd = e.X
yEnd = e.Y
End Sub
End Class
I tried making changes but i can't load the image i want into the picturebox4 and draw on it , it always loads a white background as for the undo function it works until a click again on picturebox4 and all the undone drawings appear back. Can someone help me fix this 2 problems that I have?