(Sorry, I don't know how to post the images here).
Click the 'Reply' button (or 'Edit Reply' to change a previous post)
Then, below the box where you make the new (or edited) post is a pretty small link called 'File Manager'
If you open that link you can attach images to the forum post.
You have to select the file, then 'add' the file, then click 'done adding files'.
But after a few clicks and doing it once or twice it gets easier. See below.