Two options:
1) in the additional options box within the web page below the box where you type your post, click on manage attachments. Click browse and then drag the photo from iPhoto to the file requestor. Then click the upload button, and the picture will appear as an attachment to your post.
2) Get an account at flickr.com, imageshack.com, photobucket.com, etc, etc, etc, and post the image there. Then you'll get a URL, and you can click on the yellow icon above the text box with the mountains and the sun picture, and then paste in the URL. But you need to use a site such as those three that will allow you to directly link (e.g. no snapfish or MSN photos...)