I strongly suspect you’re seeing avatars, not posters. There’s no way I could find to count posters other than tapping each contact card with an avatar in the list and seeing if you get a full-screen poster view (like the rightmost screenshot above). It took me quite some time to find the four or five people in my contacts who have set posters of themselves.
Because it’s so hard to count received contact posters, I’m only asking about how many you have manually set for yourself and your contacts.