说明：  asp图形验证码 验证码原理： 在生成验证码图片的时候，同时生成了一个Session，其值就是验证码图片中的数字值。 同时，提供输入框让用户输入，提交输入值后，与已有的Session值进行比较，根据判断结果做相应判断。
(asp graphics code validation code verification principle : generate authentication code in the picture, while a generation of Session, and its value is verified codes in the picture digital value. While providing input box allows users to input, submitted the import value, and there's value Session, According to judge the outcome accordingly judgment.)