Firstly, add data-images="image1.jpg,image2.jpg"
attribute to the image tag
<div class="sp-slider">
<img class="sp-slider__image" data-index="0" data-images="images/image1.jpg,images/image2.jpg,images/image3.jpg,images/image4.jpg" src="images/image1.jpg" alt="Image">
</div>
Then, initialize by passing the HTMLElement object to initSpSlider.
Ex.
initSpSlider(document.querySelector('.sp-slider__image'));
or for multible elements...
const images = document.querySelectorAll('.sp-slider__image');
images && images.forEach(item => {
initSpSlider(item);
});