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.