index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Image Transition Site</title>
    <link rel="stylesheet" href="style.css">
</head>
<body>
<div class="container">

    <div class="img-box">
        <img src="images/transparent.png" width="100%">

        <div class="img-wrap">
            <img src="images/original.jpg" id="originalImg"> 
        </div>

        <span id="line">
            <img src="images/arrow.png" class="arrow">
        </span>
    </div>
</div>
<script>
    var ImgBox = document.querySelector(".img-box");
    var ImgWrap = document.querySelector(".img-wrap");
    var originalImg = document.getElementById("originalImg");
    var line = document.getElementById("line");


    originalImg.style.width = ImgBox.offsetWidth + "px";
    var leftSpace = ImgBox.offsetLeft
    ImgBox.onmousemove = function(e){
        var boxWidth = (e.pageX - leftSpace) + "px";
        ImgWrap.style.width = boxWidth;
        line.style.left = boxWidth;
    }
</script>
</body>
</html>