First of all, the image must be hosted on a web server. You can't upload images to this site. Lots of folks seem to use photobucket.com, or if you have your own web site you can host pictures there.
In the "Reply" and "Post a New Topic" screens, there is a collection of buttons above the box where you enter your text. If you click on Img, you'll get a pair of tag that goes at the beginning of your picture's URL. Type (or cut and paste) the URL after that img tag, and then click the button again for the tag to close the link. Your picture will appear as an in-line graphic.
Please be careful when linking to pictures on other web sites. I take copyright very seriously, and I will remove pictures whose use might violate copyright in my sole judgment.