EventImage.php

<?php

namespace App\EventBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Illarra\ContentBundle\Entity\Image as BaseImage;

/**
 * @ORM\Entity(repositoryClass="Illarra\ContentBundle\Repository\Image")
 * @ORM\Table(name="event_image")
 */
class EventImage extends BaseImage
{
    /**
     * @return string 
     */
    public function getImageSubdir()
    {
        return 'event';
    }
    
    // ...
}

Image.php

<?php

namespace Illarra\ContentBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\MappedSuperclass
 * @ORM\HasLifecycleCallbacks
 */
class Image
{
    use \Knp\DoctrineBehaviors\Model\Timestampable\Timestampable,
        \Knp\DoctrineBehaviors\Model\Sortable\Sortable,
        \Illarra\CoreBundle\Traits\Entity\Featured,
        \Illarra\CoreBundle\Traits\Entity\Visible;

    // ...
}