//在服务器控件的属性中,需要用<%#...>来绑定其他控件的ID, 并且要在页面初始方法中,执行Page.DataBind();
<asp:ImageButton ID="imgBtnLogin" ImageUrl="~/App_Themes/DefaultTheme/Images/login_10.png" runat="server" BorderStyle="None" OnClientClick='<%#"pwdMd5("+this.txtUserPwd.ClientID+")"%>' Style="z-index: 1; width: 80px; height: 34px; cursor: pointer;" OnClick="imgBtnLogin_Click" />