function confirmerSupprimer()
      {
        var confirm=confimrm("est vous sur de vouloir supprimer ce facturation");
        if(confimrm==false)
          return false;
      }
    </script>

<asp:TemplateColumn>
                                <HeaderStyle Width="15%"></HeaderStyle>
                                <ItemTemplate>
                                    <asp:ImageButton id="ibEdit" runat="server" CommandName="update" ImageUrl="./Images/edit.gif" AlternateText="Editer"></asp:ImageButton>
                                    <asp:ImageButton id="ibDelete" runat="server" CommandName="delete" ImageUrl="./Images/del.gif"
                                                            AlternateText="Supprimer" OnClick="confirmerSupprimer();"></asp:ImageButton>
                                </ItemTemplate>
                                <EditItemTemplate>
                                    <asp:ImageButton id="ImageButton6" runat="server" CommandName="update" ImageUrl="images/save.gif"
                                        AlternateText="Valider"></asp:ImageButton>

                                    <asp:ImageButton id="ImageButton4" runat="server" CommandName="cancel" ImageUrl="./Images/cancel.gif"
                                        AlternateText="Annuler"></asp:ImageButton>
                                </EditItemTemplate>

<asp:DataGrid OnItemDataBound="yourDataGrid_RowDataBound"

protected void yourDataGrid_RowDataBound(object sender, DataGridItemEventArgs e)
{
    if (e.Item.ItemType != ListItemType.AlternatingItem && e.Item.ItemType != ListItemType.Item) return;

    ImageButton ib = e.Item.Cells[YourIndex].FindControl("ibDelete") as ImageButton;
    ib.Attributes["onclick"] = "javascript:return confirmerSupprimer()";
}