TOPIC: Security Captcha image not shows

Hi, at beginning sorry for my english i write from italy,
i have a problem with the captcha image that don't show. I have the GD library enabled and all the file permissions are ok, as i read in other posts.
The strange thing is that, the site is at this address www.ilmionido.com , in another folder www.ilmionido.com/portal i installed, previously, another joomla site (same version and components) and here the captcha is working.
Is some one able to help me!

Hey there please turn the captcha on, so I can see why it doesn't work.

Done!!!

you get

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, This email address is being protected from spambots. You need JavaScript enabled to view it. and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Look in the error log to get more information why this problem occurs.
To see most possible reasons and solutions search the forum.