Измените

 

Warning: imagepng(/var/www/mojaopstina.rs/www-root/public/images/captcha/21ac93d5f4b8c1c415253d4f15dd1f2d.png): failed to open stream: Permission denied in /var/www/mojaopstina.rs/www-root/library/Zend/Captcha/Image.php on line 563