Required helper includes were not included in the form builder when the image_tag helper was referenced from simple_captcha_image.
simple_captcha submits the captcha key (for the DB entry) in the controller based method in the session only. In the model based method, it's submitted in a hidden field. This patch submits the captcha key in a hidden field in the controller based method, too, so it works without cookies.
I am using Ruby 1.9.3 and Rails 3.2.3, i used your gem but captcha image is not displaying,its giving error StandardError (Error while running convert: convert: unrecognized gravity type `'Center'' @ convert.c/ConvertImageCommand/1508. ): and then i realised 'center' in line no. 67 causing this error.so i making this change.