If my dog can send me photos then I'll enter it on America's Got Talent. Or maybe it will be smart enough to enter itself. Or did you mean a photo of a dog? In which case I assume you mean the use of tags. Photos don't tag themselves and until image analysis technology is good enough to auto tag photos then organising through folders is good enough for me. For any application that deals with collections of stuff (music/photos/etc) the use of folders as an organisation aid has got to be the most fundamental requirement. After you got that sorted then go for the fancy stuff like tags and artificial semantic reasoning and deduction logic etc.