I think many people will have this question. On the tool bar, there is an icon of "insert image". If you already have your photo uploaded in some other website, such as flickr, all you need to do is copy the the image url of the image and past it in the post. Here are the steps:
1. Click on the icon of "insert image", so there will be image code [ img][/img] in your post
2. Go to your photo and copy image url, example http ://l.yimg.com/g/images/spaceball.gif
3. Insert the link between the code [ img][/img], like [ img]http://l.yimg.com/g/images/spaceball.gif[/img]
That is it! You can preview it just to make sure it works.