Gallery Model:

<?php
namespace Foo\Bar\Domain\Model;

use TYPO3\Flow\Annotations as Flow;
use Doctrine\ORM\Mapping as ORM;

/**
 * A gallery
 *
 * @Flow\Entity
 */
class Gallery {

    /**
     * The images of this gallery
     *
     * @var \Doctrine\Common\Collections\Collection<\Foo\Bar\Domain\Model\Image>
     * @ORM\OneToMany(mappedBy="gallery", cascade={"all"}, orphanRemoval=true)
     */
    protected $images;

    public function __construct() {
        $this->images = new \Doctrine\Common\Collections\ArrayCollection();
    }

   ....

}

?>

Image Model:

<?php
namespace Foo\Bar\Domain\Model;

use TYPO3\Flow\Annotations as Flow;
use Doctrine\ORM\Mapping as ORM;

/**
 * @Flow\Entity
 */
class Image extends \TYPO3\Media\Domain\Model\Image {

    /**
     * The gallery this image belongs to
     *
     * @var \Foo\Bar\Domain\Model\Gallery
     * @ORM\ManyToOne(inversedBy="images")
     * @ORM\Column(nullable=true)
     */
    protected $gallery;

    /**
     * @return object
     */
    public function getGallery() {
        return $this->gallery;
    }

    /**
     * @param $gallery
     * @return void
     */
    public function setGallery($gallery) {
        $this->gallery = $gallery;
    }

    ...
}
?>