[Nexus]

<button type="button" id="change-wallpaper">Change wallpaper</button>
<script>
    (function() {
        const wallpapers = [
            'https://www.cats.org.uk/uploads/images/featurebox_sidebar_kids/grief-and-loss.jpg',
            'https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/hires/2017/docatspurrwh.jpg'
        ];
        let current = -1;
        document.querySelector('#change-wallpaper').addEventListener('click', function() {
            if (!wallpapers[++current])
                current = 0;

            document.body.style.backgroundImage = 'url(' + wallpapers[current] + ')';
        });
    })();
</script>