public static class CaptchaHelper
    {
        public static string Captcha(this HtmlHelper helper, string text)
        {
            string srcPath = System.Web.VirtualPathUtility.ToAbsolute("~/Handler1.ashx");
            string htmlContent = string.Empty;
            htmlContent += "<script type=\"text/javascript\">function __rc(){document.getElementById(\"" + text +
                           "\").src = \"../Handler1.ashx?query=\" + Math.random();}</script>";
            htmlContent += string.Format("<img id=\"{0}\" src=\"{1}\" alt=\"Captcha Image\"/>", text, srcPath);
            htmlContent += "<a href=\"#\" onclick=\"javascript:__rc();\">Reset</a>";
            return htmlContent;
        }
    }

And in the View you can use like, you may need to import the Namespace of CaptchaHelper class.

 < %= Html.Captcha("Sample") %>

It will render a security image. Happy Programming

using (Bitmap b = new Bitmap(150, 40, PixelFormat.Format32bppArgb))
        {
            using (Graphics g = Graphics.FromImage(b))
            {
                Rectangle rect = new Rectangle(0, 0, 149, 39);
                g.FillRectangle(Brushes.White, rect);

                // Create string to draw.
                Random r = new Random();
                int startIndex = r.Next(1, 5);
                int length = r.Next(5, 10);
                String drawString = Guid.NewGuid().ToString().Replace("-", "0").Substring(startIndex, length);

                // Create font and brush.
                Font drawFont = new Font("Arial", 16, FontStyle.Italic | FontStyle.Strikeout);
                using (SolidBrush drawBrush = new SolidBrush(Color.Black))
                {
                    // Create point for upper-left corner of drawing.
                    PointF drawPoint = new PointF(15, 10);

                    // Draw string to screen.
                    g.DrawRectangle(new Pen(Color.Red, 0), rect);
                    g.DrawString(drawString, drawFont, drawBrush, drawPoint);
                }
                b.Save(context.Response.OutputStream, ImageFormat.Jpeg);
                context.Response.ContentType = "image/jpeg";
                context.Response.End();
            }
        }

I wrote the code in the Process Request event in HTTPHandler.
And to use this in your pages you can create an IMG tag with src attribute pointing to this.

<img src="myhandler.ashx" />

Note: Some time ASP.Net caches the image, so you may need to pass some GUID as querystring in the Handler.

Update: Sometime we may need to refresh the Captcha image, without post back, here is a simple javascript which will refresh the captcha image without post back.

<script type="text/javascript">
        function RefreshCaptcha() {
            var img = document.getElementById("imgCaptcha");
            img.src = "Handler.ashx?query=" + Math.random();
        }
</script>
<div>
<img src="Handler.ashx" id="imgCaptcha" />
<a href="#" onclick="javascript:RefreshCaptcha();">Refresh</a>
</div>

Here is the screenshot of web page using Captcha.

Captcha - Security Image using ASP.Net and C#