There is an Insert button on the top. When you click it, it will have an Image button. Click that. Then, put the direct link in "Source"

Voila! You've inserted a picture.

Holly~

Ignore.

A direct link is the link you copy from an image hosting site. So, if you upload a picture to postimage, you'll be able to copy the direct link. Once you do, you can paste that link here. Go to the insert tab, and click either "image..." if you want just the image displayed, or click "link..." if you want a clickable link displayed.

If you're talking about a link to a forum, wolf, etc. Copy the link to the page (hold down on the link, then hit copy) and paste it in the "link..." section of "insert" at the top