cloverfieldparasite.blogspot.com has an interesting article on how the poster got leaked. I’m not going to post the exact story (you can check out their site for that) but basically a woman who is known for selling movie posters and bootleg movies somehow stole the poster and was selling it at Franks And Sons. Apparently Paramount has already gotten to her and she’s probably going to get in some big trouble.
UPDATE: Earlier I had some information up here that was all speculation as to how the original hi-res pictures originated. One of my theories traced it back to a user on the unfiction forums, luwho1234, turns out this person has absolutely nothing to do with the vendor and was indeed just a fan taking pictures. I apologize to luwho, sorry for any backlash it may have caused.
*Note* – This is information that cloverfieldparasite gathered and as they also stated, it may or may not be true.