After hash values of suspect images were flagged by the private vendor, subsequent opening of the files by law enforcement did not violate the Fourth Amendment, as the opening didn't expand the scope of the search.
http://www.ca5.uscourts.gov/opinions/pub/17/17-41116-CR0.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.