change warning traingle to an information icon

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ASP.NET

Hi,i wanted to change the warning icon in my pop up message to an information icon..Can anyone please help me
<asp:LinkButton ID="lnkForgtnPwd" runat="server" CssClass="messageLogIn" OnClick="lnk_click" OnClientClick=" ShowMessage()">Forgotten your id?

I have created the link "Forgotten your id?".On clicking this link its calls the function ShowMessage()

function ShowMessage()
      {
          alert("Contact the admin")
          return false;
      }

Contact the admin is displayed as a pop up message ,but its displayed with a warning icon.i want it with an information icon..