You can determine when there may be an icon change by checking if the update_time on an entry is newer than at the previous startup/check, that would indicate that there has been a change that rebuilt the database entry and it may have generated a new .png icon.
You can determine when there may be an icon change by checking if the update_time on an entry is newer than at the previous startup/check, that would indicate that there has been a change that rebuilt the database entry and it may have generated a new .png icon.