The easiest way to post pics is to upload them to an online image service. Many are free. I use Picassa.
Once you have your images uploaded, select the the one you want to link. (make sure you have the actual image and not it's miniature version/thumbnail). Right-click the image and select "copy image location". Now in your forum post, click the little icon that looks like a landscape painting and hit ctrl+v to paste the copied URL to the field and click "OK".