duplicate background images in asp.net menuitem

I am trying to change the look of the asp.net menu by setting a background image in my stylesheet but in the menu I get duplicate backgrounds like a separate background for the text in that menu item. I have written the following in the stylesheet:

.menuitem
{
      background-image:url(./images/menu_bg.gif);
      width:180px;
      height:25px;
      color:White;
      background-repeat:no-repeat;
}

and the source for the menu looks like this:

<asp:Menu ID="Menu1" runat="server">
                            <LevelMenuItemStyles>
                                <asp:MenuItemStyle BackColor="White" CssClass="menuitem" Font-Underline="False" ItemSpacing="5px" />
                            </LevelMenuItemStyles>
                            <Items>
                                <asp:MenuItem Text="Overview" Value="Overview"></asp:MenuItem>
                                <asp:MenuItem Text="Contacts" Value="Contacts"></asp:MenuItem>
                            </Items>
                        </asp:Menu>

Am I missing something?