Flickr API for location based images

I want to get images of cities when city name is entered in search field in iOS. I am using flickr API but whenever I enter any longitude and latitude values, I only get returned an empty array. What should I do. I am using this URL:

http://api.flickr.com/services/rest/?method=flickr.photos.geo.photosForLocation&api_key=e3d577010e5979a2ad2a22714abd901e&lat=40.6700&lon=73.9400&format=json&nojsoncallback=1&auth_token=72157638668602974-e1a3a3aa1e6d3dd8&api_sig=a0233b016c863b1662aeb21a664c351a

Please tell me what should I do. any help appreciated