Hello @PixelBuddy,
Thanks for writing to us.
It seems that the image close.png is not there in the JS_composer plugin. I would suggest you deactivate and delete the plugin and then reinstall the plugin. Please make sure you have a backup of the site files and database before performing this task.
OR
It might be the issue of file permission issue I would suggest you access the file with the help of FTP and go to this file path wp-content/plugins/js_composer/assets/lib/bower/lightbox2/dist/images, check for file permission. Please make sure you have the correct file permission.
Hope it helps
Thanks