Thanks for posting the link bikerchick but that provides directiosn to use attachments
To post on the web you will have to get a website that allows hot-linking. Freeyellow now makes you pay for their site so my usual site is gone and I no longer have one.
So for starters you will have to find a web page that allows you to post pictures from their web site.
Format your images as either jpg or gif (jpgs are smaller and best of pictures gif is better for line drawings (like emoticons/smiles)
Once you have your website that allows the hot linking then upload your images to the website and open the page the image is on.
Copy the URL from the address bar and paste it into the little mountain scene icon on the posting toolbar (the one before the smile face)
Of course it seems that if you are running a mac you might not see the icons.