Is this grammatically correct? omitting "and"
Solution 1:
As Greybeard says, in your sentence people are posting the cultural sites and monuments online. They're not. They are posting clips, films or movie clips. You need to rearrange it.
I think this would be the most precise way to say it:
It is terrifying to see, posted online, recordings people have made of themselves demolishing cultural sites and monuments.
Or possibly:
It is terrifying to see recordings people have made of themselves demolishing cultural sites and monuments being posted online.