To use a custom overlay on thumbnails, add a custom overlay file to the project, below. Then indicate the album objects to which this custom overlay should be applied - see Image & Folder Settings.
The text that is displayed, which defaults to NEW, is set under Text » Index Page. Its size and style can be adjusted under Fonts » Index Page. When a visitor hovers on the "New" icon, the popup tooltip shows the date on which the image was added or taken. The tooltip prefix, which defaults to Added: or Taken:, depending upon the "newness" criterion chosen, is set under Text » Index Page. The date format is taken from the jAlbum setting under Album » Settings » Advanced » General.