Photos on the internet should be viewed with caution, unless you can find a site which one would expect to show reliable photos.
Hymenocallis littoralis on the Smithsonian Tropical Research Institute I would consider more likely to be reliable than not.
http://biogeodb.stri.si.edu/bi...
The long tepals and size seem to match this, however one can never be certain. The length of the tube suggests a large flower but we can't see the tube in the photo posted.
Each flower's tube is 14 to 17 cm (5 to 7 inches) long or longer
http://en.wikipedia.org/wiki/H...
