When you create a new post, there's a row of icons across the top of the text editor window. The tree icon is the Insert Image button.
I would advise against linking directly to large images. The forum limits image size to 600 x 600 pixels to protect the page formatting, so large pictures tend to get distorted. Also, you don't want people to be unexpectedly downloading a large file when they open a forum message.
If you want to see if your link code is working, then you can use the Previwe button below the text editor window. If your link is correct, then the image will appear in the preview display.
Nope, the pic didn't appear. :|