You need a minimum number of posts to link to other pages and edit. Until then, you can't do it. When you hit that limit, the tool bar will change. It is the 10th icon from the right. or just copy and paste the link.
Upload your image or file to a web hosting site. Google to find a free hosting site. Provide a link to the image on that hosting site and you're done.
If you use TapaTalk, the S/W will do this for you, so there is no need to link.