It's a spammer AFAIK, I considered the possibility of it being used to get IP's and deanonimize users through the image but it seems this isn't the case since the image points to a generic pictrs file from a Lemmy instance and most clients / frontends proxy image loading.
...is it blockchain based? or is there a central server somewhere orchestrating it all ?
It does not use blockchain or a central server, but instead uses the ActivityPub protocol, just like Mastodon. That's why Mastodon users can see some Lemmy posts and the other way around.
EDIT: The post has been restored after OP send a verification video of them holding up a piece of paper with 10 random alphanumerical characters as requested
I guess I'll explode