Below is an example of the Captcha Control

Download the complete solution from CodePlex.

Captcha Control added to ASP.NET Page.

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//Captcha Control
<cc1:CaptchaControl ID="myCaptchaControl" ClientIDMode="Static"
  Width="800"
  Height="75"
  ImageWidth="400"
  ImageHeight="75"
  Minimum="5"
  Maximum="6"
  CaptchaHandlerUrl="~/CaptchaHandler.ashx"
  Overlap="6"
  RefreshButtonPath="~/Images/refresh.png"  runat="server" />

<asp:PlaceHolder ID="PlaceHolder1" runat="server"></asp:PlaceHolder>

Please enter characters displayed above&nbsp;

<asp:TextBox ID="CaptchaCharacters" ClientIDMode="Static" Text="" Width="100" ToolTip="Enter characters displayed above" runat="server">
</asp:TextBox>

<asp:RequiredFieldValidator runat="server" ValidationGroup="RegisterGroup" ontrolToValidate="CaptchaCharacters" CssClass="field-validation-error" ErrorMessage="*Required" />

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Captcha Control will render out a HTML image element (image displaying characters) and a HTML input element of type button (refresh button - retrieving a new set of display characters).
Below the Capthca Control, I added a Textbox, which is where the user would type the Captcha characters and will be verified once the form is submitted.

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Code Behind - Verifing Captcha Characters

Note: Captcha control had been place within a CreateUserWizard control. Referencing Captcha Control, I must FindControl within CreateUserWizard control (ID="RegisterUserWizardStep").

 //CreatingUser is raised before the Memebership Provider's CreateUser. Setting LoginCancelEventArgs Cancel property equal to true, the user will not be created.
protected void RegisterUser_CreatingUser(object sender, LoginCancelEventArgs e)
{
    var err = ((Literal)GetControl("ErrorMessage"));
    var parent = err.Parent;
    err.Text = "";

    Page.Validate();
    if (!Page.IsValid)
    {
       err.Text = "Required fields are empty or incorrect information was entered.";
       e.Cancel = true;
     }
     else
     {
       //Initiate the EncodeCaptcha class
       var ec = new EncodeCaptcha(GetCaptchaKey());

       //GetCaptchaCharacters returns clear Captcha characters represented in the displayed image.
       //Test they must be equal
       if (ec.characters != GetCaptchaCharacters())
       {
          err.Text = "Characters incorrectly entered. Please reenter correct characters displayed";
          e.Cancel = true;
       }
      }
}

//Return submited Captcha characters. the characters that the user had entered.
string GetCaptchaCharacters()
{
    return ((TextBox)GetControl("CaptchaCharacters")).Text;
}

//Return encrypted Captcha character string. The encrypted string had be UrlEncode, so the return
//string is UrlDecoded.
string GetCaptchaKey()
{
    var cc = GetControl("myCaptchaControl");
    var captchaCharacters = (HiddenField)cc.FindControl("Captcha-Key");

    return HttpUtility.UrlDecode(Request[captchaCharacters.UniqueID]);
}

//Abstracted findng fields within CreateUserWizard.
Control GetControl(string value)
{
    return RegisterUserWizardStep.ContentTemplateContainer.FindControl(value);
}