As a brand new user with limited experience, I had the same question. It appears to work properly after I made the following edits to two php files.
In jwsig.php, found in public_html/plugins/content/, at line 99 I replaced "stories" with the name of my preferred folder.
$rootfolder = '/images/stories/';
And in showthumb.php, found in public_html/plugins/content/plugin_jw_sig/, at line 29 I replaced "stories" with the name of my preferred folder:
$_image_ = '../../../images/stories/'.$_GET;