Hello,
wp-content/plugins/et-core-plugin/app/controllers/shortcodes/banner.php
But you will lose changes after every Core plugin update.
Here you can post a request about changing it globally in one of next our updates, or about adding a new option for the Banner element – https://www.8theme.com/taskboard/ If this request takes votes also from other customers then our developers will be able to implement this.
Regards