Two more points I wanted to make.
One is that Hezek Re’iyah has nothing to do with this. As opposed to the common misunderstanding of the term, it is a Machlokes Rishonim and refers either to Ayin Hara, or to someone feeling uncomfortable and self conscious since he is being watched. Well, on second thought, perhaps there is some connection. But only to Ayin Hara, possibly – the second approach is only b’Shaas Maiseh.
Also, the lack of Tznius is applicable whether or not the Chosson and Kallah are behaving inappropriately. As others have noted, that which they are at the event of their own wedding, in the presence of their guests, is entirely appropriate, and if there are those in attendance who abuse their Shemiras Einayim then it is not (necessarily) the fault of the Chosson and Kallah. Whereas posting a picture for any and all to see – including those who have no connection to the Simchah – is inviting the voyeur in many. Especially considering that there does not seem to be any compelling reason to do so.