SOURCE

let lazy_list = document.querySelectorAll("img.lazy");

window.addEventListener("scroll",()=>{
    lazy_list.forEach((img_obj)=>{
    if(document.documentElement.scrollTop+window.innerHeight>img_obj.parentElement.offsetTop){
        img_obj.src = img_obj.getAttribute("data-src");
        img_obj.classList.remove("lazy");
        lazy_list = document.querySelectorAll("img.lazy");
    }        
})

})

<ul>
	<li><img class="lazy" data-src="https://s3.jpg.cm/2020/10/28/NgstC.jpg" alt=""></li>
	<li><img class="lazy" data-src="https://s3.jpg.cm/2020/10/28/NgkLR.jpg" alt=""></li>
	<li><img class="lazy" data-src="https://s3.jpg.cm/2020/10/28/Ngr55.jpg" alt=""></li>
	<li><img class="lazy" data-src="https://s3.jpg.cm/2020/10/28/NgE0t.jpg" alt=""></li>
	<li><img class="lazy" data-src="https://s3.jpg.cm/2020/10/28/NgmUz.jpg" alt=""></li>
</ul>

*{
    margin: 0;
    padding: 0;
    list-style: 0;
}
li{
    width: 50%;
    padding-top: 28%;
    overflow: hidden;
    position: relative;
    margin: 50px auto;
    background-color: #ccc;
}
img{
    position: absolute;
    width: 100%;
    top: 0;
    left: 0;
}