Here's an in-depth guide I made, but it's wordy and drones on so I don't blame you if you want a tl;dr.
The Boards uses a limited version of Markdown,
Using "---" or "___" gives a:
Using a "#" at the beginning gives:
#A Header
Using "######" at the beginning gives:
######A sub-text
As far as pictures and gifs go that's actually really easy, it's called embedding. I can't go all technical into it because, well, I don't know the fancy background stuff behind it. But to use it just take the direct URL (usually end in".jpg" for pictures and ".gif" for Gif's) and copy it into your post body. It'll only show up after you post though, and it'll take a second to pull the file so don't panic ^_^
You can actually embed other things too, like youtube videos, soundcloud files and entire Imgur albums that same way. More too, I have a list in that first link.
Hope that helped! Feel free to ask for anything else, I'll help best I can. Although I may be delayed until tomorrow as it's getting late where I am.