This notice appear because of your PHP version ( most likely your PHP version is lower then 5.3).
1. Find out your version of PHP using steps 1,2,3 under "GD Library" section of this manual:http://docs.oxwall.org/faq:captcha-troubleshooting
2. If your version of PHP is lower then 5.3, check whether your hosting company can update it for you.
If your host can not update PHP version to 5.3, open error_manager.php mentioned in your error notice and add "//" before following line:
case E_DEPRECATED:
So overall you should have:
//case E_DEPRECATED: