document.querySelectorAll('.slider__itm img').forEach(n => {
const link = document.createElement('a');
n.parentNode.append(link);
link.append(n);
});
или
for (const n of document.querySelectorAll('.slider__itm')) {
n.innerHTML = `<a>${n.innerHTML}</a>`;
}
или
for (const n of document.getElementsByClassName('slider__itm')) {
const link = document.createElement('a');
link.appendChild(n.replaceChild(link, n.children[0]));
}
или
const imgs = document.querySelectorAll('.slider__itm img');
for (let i = 0; i < imgs.length; i++) {
const link = document.createElement('a');
imgs[i].replaceWith(link);
link.insertAdjacentElement('afterbegin', imgs[i]);
}