function.mediaById.php

<?php
/**
 * Turn a media-id into a valid path
 * @param $params
 * @param $template
 * @return string
 */
function smarty_function_mediaById($params, $template)
{
    $id = $params['id'];
    $size = $params['size'] ?: false;

    $mediaRepository = Shopware()->Models()->getRepository('Shopware\Models\Media\Media');
    $mediaService = Shopware()->Container()->get('shopware_media.media_service');

    $mediaObject = $mediaRepository->find((int)$id);

    if (!$mediaObject) {
        return $id;
    }

    if ($size) {
        $thumbnails = $mediaObject->getThumbnails();
        $thumbnail = $thumbnails[$size] ?: array_values($thumbnails)[$size];

        if ($thumbnail) {
            return $thumbnail;
        }
    }

    // turn object into path
    return $mediaService->getUrl($mediaObject->getPath());

}

usage.tpl

<img src="{mediaById id=152322}">
<img src="{mediaById id=152323 size=1}">
<img src="{mediaById id=152324 size=80x80}">