refer my article 

http://www.c-sharpcorner.com/UploadFile/ca9151/generation-of-captcha-image-using-generic-handler/

refer 
http://dotnetlearners.com/blogs/view/6/Create-your-own-captcha-image-generator-in-aspnet-using-c-net.aspx 
http://www.aspdotnet-suresh.com/2013/09/create-captcha-with-refresh-button-in-aspnet.html 
http://www.ultimateprogrammingtutorials.info/2012/12/how-to-make-simple-captcha-control-in.html

In .aspx page:

captcha image

   
       
                   


           
           

Prove You are not a robot:		Font-Size="X-Large" Font-Bold="True" BackColor="#CC99FF" ForeColor="#CC0066"                          Width="148px" Height="33px">	ImageUrl="~/images/14635845-recycling-reload-white-transparent-and-glossy-web-interface-icon-with-reflection.jpg"                  Width="30px" Height="30px" AlternateText="refresh"                  onclick="ImageButton1_Click" />	ImageUrl="~/images/download.jpg" Height="30px" Width="30px"                      onclick="ImageButton2_Click"/>	ControlToValidate="TextBox1" ControlToCompare="txtrobocomp"                              ErrorMessage="Please enter correct Text" ForeColor="Red">
style="height: 26px" />

In .aspx.cs page:
 protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                StringBuilder builder = new StringBuilder();
                builder.Append(RandomNumber(10, 99));
                builder.Append(RandomString(3, true));
                builder.Append(RandomString(2, false));
                txtrobocomp.Text = builder.ToString();
            }
        }
        private int RandomNumber(int min, int max)
        {
            Random rd = new Random();
            return rd.Next(min, max);
        }


        private string RandomString(int size, bool lowerCase)
        {
            StringBuilder builder = new StringBuilder();
            Random rd = new Random();
            char ch;
            for (int i = 0; i < size; i++)
            {
                ch = Convert.ToChar(Convert.ToInt32(Math.Floor(26 * rd.NextDouble() + 65)));
                builder.Append(ch);
            }
            if (lowerCase)
                return builder.ToString().ToLower();
            return builder.ToString();
        }


        protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
        {
            StringBuilder builder = new StringBuilder();
            builder.Append(RandomNumber(10, 99));
            builder.Append(RandomString(3, true));
            builder.Append(RandomString(2, false));
            txtrobocomp.Text = builder.ToString();
        }


        protected void ImageButton2_Click(object sender, ImageClickEventArgs e)
        {
            SpVoice sp = new SpVoice();
            sp.Volume = 90;
            sp.Speak(txtrobocomp.Text);
            //sp.WaitUntilDone(Timeout.Infinite);
        }

I hope it will help you

Thanks& Regards
Aswini Aluri

hi,

refer to this link

http://www.codeproject.com/Articles/169371/Captcha-Image-using-C-in-ASP-NET

http://www.codeproject.com/Articles/99148/Simple-CAPTCHA-Create-your-own-in-C