Need user driven filter menu capability for your photo gallery.  For example, Picassa lets you create tags or key words as well as title. 
1) You should support generic search capability for description/title of a photograph
2) You should offer multilevel filter capability so you can sort by category and/or sub category for a particular grouping of photos/images.  It would be nice to sort on more than ONE dimension or element.  For example, I might want to filter on a product category and I also might want to sort on a timeframe (mo/year/decade/etc.)
          I believe there are a lot of uses for this photo database filtering capability.  It is UNREASONABLE for anyone to assume that a USER will KNOW what categories and tags you have in your database.   You need to be able to display them so they know what their OPTIONS are upfront.   I have not seen this capability anywhere.  I have been designing web applications almost since the INTERNET began (I am NOT a programmer) but our Thomas & Betts web order entry system has generated over ONE BILLION DOLLARS in orders for the company. 
           Sorting photos by face recognition and/or object recognition, GPS, time stamps, etc are all neat BUT they do not provide a clean consistent categoration of information.  Photos are NOT necessarily random images ... they need to be indexed and cataloged in a logical way so that someone can quickly view precisely what they are looking for.  Hope you will give this idea some consideration as I believe you would find it to be financially beneficial for a number of business and historical reference applications.