This clean-up script searches through your image upload folder (default "user_images") for any image filenames that are no longer referenced in the database (and therefore no longer attached to listings).
In most cases, the functionality provided by this script occurs naturally as each individual listing expires from the site, so running this is normally not needed. However, it may help free up disk space on some server configurations, especially older sites or those with lots of uploaded images.
Since this involves deleting files completely, there is no "undo" button, so exercise caution when running this script.