HOW TO CHANGE THE IMAGE SOURCE OF AN IMAGE BUTTON ON MOUSE OVER

  Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

imgBtn_Rename.Attributes.Add("onmouseover", "this.src='images/icons/rename2.gif';")
imgBtn_Rename.Attributes.Add("onmouseout", "this.src='images/icons/rename.gif';")

  End Sub

<asp:ImageButton ID="imgBtn_Rename" runat="server" ImageUrl="~/images/icons/rename2.gif"
                    AlternateText="Rename" Height="20px"></asp:ImageButton>