State of Shock

well just take a look at arab social media and read their posts.

yes those photos are real
and yes both photos and social media posts have to do with another.
both dislplaying flagrant cheer

source:

pic 1 footage has been taken and tweeted out by this frenchman
https://twitter.com/flori_tori
I did an edit by moving the front left head slightly to the right in order to crop the picture.

pic 2 is from a news outlet (here's a variation)

pic 3 already gone viral - don't know the original source (probably some frenchman)
I picked it up from Amy Mek's twitter
https://twitter.com/AmyMek

well the Frenchman in the background looks extremely pissed-off and angry at those two Arabs.
looks pretty much authentic to me.
prove me wrong then I'll pull my meme.

I agree with you

Given that there's been no terrorists taking credit, I'm unsure if it's Islamic terrorists (I'd sooner suspect militant atheists).

if someone gets identified and taken in custody the common narrative will be 'lone nut with mental issues'
https://www.nbcnewyork.com/news/local/Man-in-Custody-After-Bringing-Gas-Cans-Into-St-Patricks-Cathedral-Sources-508730871.html

Most likely, unless they openly declare their intentions (like the terrorist attack in France where ISIS terrorists murdered a Catholic priest in front of his congregation then did Islamic services right there. When the police arrived to take them in the terrorists fought them and were gunned down).