I remember walking through Philly and seeing these in the 90s or early 2000s. They were very distinctive and kind of a local landmark
https://toynbeeidea.com/where2-2/ has a map of a lot of them. All the originals are long gone and only a handful remain. I would suspect at least a few that remain are possibly copycats given they came about many years later, look kind of different, some came about very far away, and the whole thing has been popularized quite a bit over the past 15ish years (there’s literally a documentary on it). The documentary maker had a pretty credible theory on who did it too iirc but I don’t remember the details
There are 4 types of posts currently on kbin.
New post is the simplest one, kinda a tweet-like message.
New photo and new link are pretty self-explanatory, and they make your header link to an image or a website respectively.
New thread is the most feature-rich post available, where you actually have a substantial body field to write your content in.
And all types of posts (and comments) can apparently have a cover image (just one tho, no galleries as far as I know).
Could be simplified a bit, if you ask me, but there’s the gist of it. So overall it’s pretty straightforward: use photo to upload a pic, use link to link to some website, and thread is for everything else that doesn’t fit those two.