Измените

 

Warning: imagepng(/var/www/mojaopstina.rs/www-root/public/images/captcha/51c9e11d1c144e5f0d2c0b94a3fede0e.png): failed to open stream: Permission denied in /var/www/mojaopstina.rs/www-root/library/Zend/Captcha/Image.php on line 563