First, you probably have to resize it to fit the 244Kb limit for attachments: you can do this either in Photoshop or with Graphic Converter, which is free. Then save it to your desktop. Click on "Manage Attachments" below the reply box, select the file from your Desktop, and click "Upload". When it has uploaded, Submit.
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...)
Closer. When you are logged in and go to that photo, and then click the all sizes button above it. Then select the size you want, and at the bottom of the page will be a box with a URL that actually ends in .jpg or .gif or whatever.
When you get that link, in your message, click on the yellow and grey mountains icon above the textbox and paste that link into the popup.