Asp.net mvc ajax image loading problem

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MVC

Hello friends,
I have created an application in mvc.In that application there is a form for add details to the database.So in the form there is a captcha image .i have written the code for update image using ajax while refreshing the captcha image.But the actual problem is when i click on the refresh button some characters appears in the place of image.How can i solve this problem?
my code is given below.

<img src="/{Controller}/ShowCaptchaImage" alt="Captcha"  width="238" height="53"/>

In my controller

public CaptchaImageResult ShowCaptchaImage()
       {
           //return new CaptchaImageResult();
           Session["CaptchaImageText"] = GetCaptchaString(6);
           CaptchaImageResult ci = new CaptchaImageResult(
       this.Session["CaptchaImageText"].ToString(),
       200, 50, "Century Schoolbook");
           this.Response.Clear();
           this.Response.ContentType = "image/jpeg";
           ci.Image.Save(this.Response.OutputStream, ImageFormat.Jpeg);
           ci.Dispose();
           return ci;

Thank you developer Guys,