How to use data-image

image is not displaying. i have side bar image in my app/assets/images/sidebar-4.jpg.but did not show. tell me if there something wrong in my code.i am a beginner in ROR

dashboard.html.erb
    <div class="sidebar" data-color="purple" data-background-color="black" data-image="/assets/sidebar-4.jpg"></div>
    my image https://imgur.com/a/QikMJJS