Well they are both the same question really, as the URL for a photo contains the photo ID.
It depends what information you want about that photo. You are probably either wanting flickr.photos.getInfo or flickr.photos.getSizes. (PhotosGetInfo or PhotosGetSizes).
It really does pay to go to the Flickr API page and try to learn what the methods there do. http://www.flickr.com/services/api