Edit MainPage for your Uri
imgCaptcha.Source = new BitmapImage(new Uri("http://localhost:2918/AntiBotImage.ashx"));

and ServiceReferences.ClientConfig
<endpoint address="http://localhost:2918/CaptchaService.asmx"
                binding="basicHttpBinding" bindingConfiguration="CaptchaServiceSoap"
                contract="CaptchaServiceReference.CaptchaServiceSoap" name="CaptchaServiceSoap" />