Hi, I've added the ability to sort by date-taken, date-posted or interestingness. The default is date-posted-desc as per the flickr API documentation.
I've added a sort_by and sort_direction to the options.
I wasn't sure what naming convention you prefer, so I called the sort string 'sort_order'.
I think most people who use this option will prefer sort_by=3 and sort_direction=0 (most popular first).
Added the ability to sort (date taken, date posted, interestingness)