Web button with image added via source - image not displayed in browser

Greteings

I have a page with various button(s) - the default ASP.NET button!. The ASP source has been modified to add an image via teh background-image property. This appears to work fine - button shows with image , in designer. When I "View in Browser" ther is NO image displayed. What gives? What do I need to do?

My source:

   <asp:Panel ID="Panel1" runat="server" BorderStyle="Outset" BorderWidth="2px" Height="50px"
        Width="930px">
        <asp:Button ID="btnInsert" runat="server" Text="   Insert" Width="85px"
             Style="text-align: center; background-repeat: no-repeat; background-image: url(D:/School_Keeper_ASP/Images/insert.bmp); background-position:left center;
             right: 846px; position: absolute; top: 474px" />
        &nbsp;      
        <asp:Button ID="btnCancel" runat="server" Text="   Cancel" Width="85px"
             Style="text-align: center; background-repeat: no-repeat; background-image: url(D:/School_Keeper_ASP/Images/cancel.bmp); background-position:left center;
             left: 396px; position: absolute; top: 472px" />
    </asp:Panel>