This filter allows you to change the filesystem path to the folder that holds the marker images. The markers found in this folder are shown on the settings page in the markers section.
This example changes the filesystem path to /wpsl-markers/ inside the current theme folder:
$admin_marker_dir value will look like this: /home/user/public_html/wp-content/themes/my_theme/wpsl-markers/.
WPSL_MARKER_URI value will look like this: http://mydomain.com/wp-content/themes/my_theme/wpsl-markers/.
This filter is located in /admin/class-settings.php