Hi Irsal,
Thanks for contacting Syncfusion forums.
Please try adding Image handlers in the Web.Config file to resolve this issue. We have prepared a simple sample to showcase the working behavior of Captcha control and please download it from the following link,
F106632Captcha.zip
You may also refer the below given online UG link which gives more details on adding Captch control in an application,
http://help.syncfusion.com/ug/asp.net%20mvc/tools/documents/addingacaptchatothemvcapplication.htm
Kindly check the link and sample.
Regards,
Varalakshmi