You can do a lot of stuff they do on powerpoint or the libre office equivelant I wouldn't bother getting paid software especially (libre office is free and open source).

Just decide on something that looks good to you.
I'm not sure how big these posts are alloweed to have a maximum but it's good to use the preview buttonn at the bottom of the posting screen if you want to check how your post will turn out.

Images can't be uploaded to the server, a lot of people will upload images elsewhere. Imgur, facebook and google work (if you send something to yourself on facebook then it works as you can copy the address).