CakePHP3.x でDBに画像を保存＆表示 ref: https://qiita.com/kazamai/items/fdb6324fcb78e1865dd1

edit.ctp

        <img src="<?= $this->Url->build(["controller" => "HogeDatas", "action" => "contents", $hogeData->id]); ?> " />

HogeController.php

    public function contents($id)
    {
        $data = $this->HogeDatas->get($id);
        $this->autoRender = false;
        $this->response->type('image/jpeg');
        $this->response->body(stream_get_contents($data->thumbnail));
    }

HogeTable.php

use RuntimeException;

    public function beforeSave($event, $entity, $options)
    {
        if ($entity->thumbnail['error'] === UPLOAD_ERR_OK) {
            $entity->thumbnail = $this->_buildThumbnail($entity->thumbnail);
        } else {
            unset($entity->thumbnail);
        }
    }

    protected function _buildThumbnail($thumbnail)
    {
        $ret = file_get_contents($thumbnail['tmp_name']);
        if ($ret === false) {
            throw new RuntimeException('Can not get thumbnail image.');
        }

        return $ret;
    }