unable in getting the picture in the picture box

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VB

i am running a program in which the list of documents is visible in the listbox and the image is shown in the picture box. but when i run the code and when i select a document in the listbox ,i got its image in the picturebox but when i select another document i dint get its respective image in picturebox. the image remains static in the picturebox when i clicked the first document......
plz help me plzzz...........her's my code

-------------------------------code--------------------------------------------------

Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
     'tb_sno.Enabled = False

     tb_name.Enabled = False
     cb_school.Enabled = False
     'cb_doctype.Enabled = False
     Dim test As String = ListBox1.Text
     tb_sno.Text = ListBox1.Text.Remove(ListBox1.Text.IndexOf(" "))


     If con.State <> ConnectionState.Open Then
         con.Open()
     End If
     GroupBox3.Visible = True
     Dim dr As SqlDataReader
     Dim sid As String
     sid = ComboBox1.SelectedItem.ToString
     sid = sid.Substring(0, 9)
     ' school = ComboBox2.Text
     'dname = ListBox1.Items.Add(dr(0))
     Dim cmd As New SqlCommand("select * from studentmaster a, hbetscan b where a.studentid='" & sid & "' and a.studentid=b.studentid ", con)
     cmd.CommandTimeout = 0
     cmd.ExecuteNonQuery()
     dr = cmd.ExecuteReader
     If dr.Read Then
         tb_sid.Text = dr("studentid")
         tb_name.Text = dr("studentname")
         cb_school.Text = dr("schoolname")
         cb_doctype.Text = dr("doctype")
         Dim imgdata() As Byte = dr("image")
         Dim img As Image
         Dim ms As MemoryStream = New MemoryStream(imgdata, 0, imgdata.Length)
         ms.Write(imgdata, 0, imgdata.Length)
         img = Image.FromStream(ms, True)
         PictureBox1.Image = img
         PictureBox1.Image.Save("c:\" & "img1.jpg", System.Drawing.Imaging.ImageFormat.Jpeg)
         dr.Close()
         con.Close()
         ms.Close()
         'ms.Dispose()


     End If
 End Sub