I have spent some time researching your question to find the precedent you are looking for, however, I've been unable to find a specific case with the same facts as the one you set forth. This is not unusual because criminal
law is very fact dependent and the law involving social network sites continues to evolve.
That said, there are certain arguments that can be made from the cases and the law that does exist. Because there does not yet appear to be a case on point, arguments can be made both for and against a finding of dissemination in the scenario you describe.
Under the applicable NY statute Article 250.40 "disseminate" means to give, provide, lend, deliver, mail, send, forward, transfer or transmit, electronically or otherwise to another person."
The statute you are trying to make applicable, Article 250.60 requires that a person "intentionally disseminates such unlawful image" ie. one created in violation of Article 250.50. Your question establishes that the image was unlawfully created leaving the issue of intentional dissemination.
The definition of dissemination in the statute requires that the image be provided, sent, transmitted "to another person." The uploading of the video to Youtube would satisfy the transmission part. The issue would be whether Youtube can be considered "another person." Another issue is that the statute requires "intentional" dissemination. Which leads to the arguments:
In favor of violation of the statute: Dissemination includes uploading to the third party carrier Youtube because people run Youtube and the person transmitting is aware that there are moderators. The terms of service indicate that the videos will be accessible by persons at Youtube. Although Youtube is an entity, people run it and see the videos. It is foresseable that actual persons will view the video. Analogies could be made to other crimes, such as exposure cases, which indicate that attempts at privacy are insufficient where it is reasonably foreseeable someone may see and does see the conduct.
Against violation of the statute: You need an actual other person to have received the video, the likelihood or possibility that someone might see the video is not sufficient for a violation. The statute requires an actual human recipient. The statute also requires "intentionally" and all you have here, at best, XXXXX XXXXX dissemination because the videos were set to private and not intended to be received or viewed by another person. Any ambiguities in statutes are construed in favor of the person charged.
I'm sorry that all I can give you are arguments and not a definitive answer or citation of specific authority that you are looking for, but that appears to be the state of the law at this point.