That plus button is the one you use for inserting an image, when you click on it, a camera icon pops and when you click on that a selection window appears, you choose your pic and click open and the image will almost immediately show up on the post. The editor is a what you see is what you get, so there is no need for a separate preview because it's already in preview mode. They are currently developing a new version which is much better but they asked us not to share the link as it is still in being tested. join the discord server to become a tester https://discord.gg/G95rNZs