Untitled

class Gallery {
    constructor(thumbnail, original) {
        this.thumbnail = thumbnail;
        this.original = original;
    }

    createImage() {
        return '<img src="' + this.thumbnail '">';
    }
}

var images = new Gallery('http://www.pixic.ru/i/2081W44634K5D3r8.jpg', 'http://www.pixic.ru/i/H0n1549674h5k3C9.jpg');

window.onload = function() {
    document.getElementById('gallery').innerHTML = images.createImage();
}

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <link rel="stylesheet" href="style.css">
    <title>Gallery</title>
</head>
<body>
    <div id="gallery"></div>
    <script src="script.js"></script>
</body>
</html>