-.I directory
-The directory to search. The directory will be searched recursively,
-and all files in it are assumed to be images.
+.I directory-or-URL
+If a directory is specified, it will be searched recursively for
+images. Any images found will eligible for display. For efficiency,
+the contents of the directory are cached for a few hours before it
+is re-scanned.
+
+If a URL is specified, it should be the URL of an RSS or Atom feed
+containing images. The first time it is accessed, all of the images
+in the feed will be downloaded to a local cache directory. If a few
+hours have elapsed since last time, the URL will be polled again, and
+any new images will be cached, any images no longer in the feed
+will be expired.
+.TP 4
+.B --no-cache
+Update the cache immediately, even if it is not time yet. This
+will re-scan the directory, or re-poll the RSS feed.