It's not. Go to My Stuff, select pictures, upload the picture you want to use and then click the image icon that appears on the bar above the text, which does not appear unless you have Firefox or Internet Explorer I think.
I found internet explorer works best for posting pictures, hope this helps.
If you're using the Rich Text Editor, click on the "Source" button, then type <img src"#"> and replace the # with the image url (the image url is obtained by right-clicking/command-clicking the image you want to post to the forums and selecting Copy Image Location).
Awwwwww Yeah!
