How many of those 13,500 visitors are in possession of a Wikisposure card?
There are more problems:
Where is the uploader located?
Is he or she aware of American laws, which regulate the Californian hosts of the Wikimedia Foundation, regarding this material?
Is he or she the local laws where they are?
What if he's a 18 year old kid in Spain and his 15-year old girlfriend (age of consent in Spain is 13) gave consent to have the pictures taken?
Who does the FBI go after if it's illegal material, and the contributor isn't in USA?