FROM THE FAQ:
In order to attach a file or image to your message, you must use the Preview Post feature. First, enter your post, and check the I want to preview my post and/or attach a file box located above the "Continue" button on the post page.
On the preview post page, you will see a "Browse" button. Click browse, and find the file on your hard drive. Then, click "Continue" again to attach the file.
Your image must be 100K or less