while surely anybody has a right to request that people not post images from his or her private affair online or in a newspaper or anywhere else, just like they can make any request, it really is not very enforceable.
I find the reason given in the sign to be a bit curious. Would some random guest posting a photo of himself at the wedding, a selfie say with his good friend or cousin he has not seen in a while, or a picture of something interesting that happened at the wedding, really damage the bracha and kedusha of their marriage and of the evening? Perhaps a more specific request - not to post pictures of the chosson and kalla, or something similar - would be more sensible, and perhaps more adhered to..
One other thing - whoever initially posted this picture of the sign on line violated the request. I do not know if that was the only person to violate the request, or even if they were the first, but they posted online, to social media (it was also on Facebook), an image from the wedding. They will one day have to answer for the damage they might have caused to this poor couple's wedding...
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