If you fill an URL here, this URL will be used instead of the regular avatar URL, and you will be given an option to select a "mood" for each of your posts. A $ character in the URL will be replaced by the mood index number for each post.
Note: If the URL does not contain "$", this setting has no effect.
Note2: This does not generate any new images; it only performs pattern substitution on the URL (replacing $ by a number). You need to provide the images yourself.