Public Class frmPicScale
 
    Dim pn As New Panel With _
          {.Parent = Me, .Dock = DockStyle.Fill, _
           .AutoSizeMode = Windows.Forms.AutoSizeMode.GrowAndShrink, _
           .AutoScroll = True}
 
    Dim WithEvents pb As New PictureBox With _
            {.Parent = pn, .SizeMode = PictureBoxSizeMode.Zoom}
 
    Dim picsize As Size
    Dim picsizefactor As Double = 1.0#
 
    Private Sub frmPicScale_Load(sender As Object, _
        e As System.EventArgs) Handles Me.Load
 
        'Bild laden und Größe notieren
        Using bmp As New Bitmap(IO.Path.Combine _
           (My.Computer.FileSystem.SpecialDirectories.MyPictures, "test.jpg"))
 
           picsize = bmp.Size
           pb.Size = picsize
           pb.Image = CType(bmp.Clone, Bitmap)
        End Using
    End Sub
 
    Private Sub frmPicScale_MouseUp(sender As Object, _
        e As System.Windows.Forms.MouseEventArgs) Handles pb.MouseUp
 
        'Anzeigegröße der Picturebox/des Bildes ändern
        If picsizefactor > 0.5 Then
            picsizefactor -= 0.1
        Else
            picsizefactor = 1.0
        End If
        pb.Width = CInt(picsize.Width * picsizefactor)
        pb.Height = CInt(picsize.Height * picsizefactor)
    End Sub
End Class