Edit in JSFiddle

ndst.content.utk1.forEach(function(material) {
  if (material.filetype === 'picture') {
    const picture = document.querySelector('img')
    picture.src = `${ccloudserver}${material.filename}`
    picture.alt = material.description
    picture.style.height = '200px'
  }
})
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Moje własne ndst.online!</title>
</head>

<body>

  <img></img>

  <script src="https://api.czooosnek.cloud/ndst.js"></script>

</body>