Измените

 

Warning: imagepng(/var/www/mojaopstina.rs/www-root/public/images/captcha/6f51ba1b6ca86ab667a6005000e9b372.png): failed to open stream: Permission denied in /var/www/mojaopstina.rs/www-root/library/Zend/Captcha/Image.php on line 563