docs: public static String getImageFilename(String strFilename, String strSubDirectory)
     {
         if ((strFilename == null) || (strFilename.length() == 0))
             return null;    // A null will tell a JButton not to load an image
         if (strFilename.indexOf('.') == -1)
             strFilename += ".gif";
         strFilename = Util.getFullFilename(strFilename, strSubDirectory, Constant.IMAGE_LOCATION, true);
         return strFilename;
     }