<asp:ImageButton ID="CheckoutImageBtn" runat="server" 
                      ImageUrl="https://www.paypal.com/en_US/i/btn/btn_xpressCheckout.gif" 
                      Width="145" AlternateText="Check out with PayPal" 
                      OnClick="CheckoutBtn_Click" 
                      BackColor="Transparent" BorderWidth="0" />