To change default cover for desktop site, please upload your name your new cover as “cover.jpg” and upload and overwrite cover from this location: app/webroot/img/cover.jpg

To change default cover for mobile app, please upload your name your new cover as “cover.jpg” and upload and overwrite cover from this location: app/webroot/theme/mooApp/img/cover.jpg

please clear global cache and your browser cache before testing