Simple slider gallery created from comma-separated list of image IDs. In order to find the image ID, navigate to the Media Library and click on the desired image. Take a look at the URL field – you will notice something like: e.g. /wp-admin/upload.php?item=147 which refers to image ID.
*Note that this widget is deprecated, and it will be completely removed in future updates. We recommend switching to default WordPress Gallery widget, as it has the same features.