If you know the URL of the image and don't have it on your computer, click the "Image" button on the toolbar (in between "Smilies" and "Media") and enter the URL. The image will appear in your post.
If you have an image on your computer that you'd like to put in your post, click the "Upload a File" button below the editor (between "Preview..." and "More Options..." This will allow you to upload the image. Once it is uploaded, select "Full Size" to show the full-sized image within your post or "Thumbnail" to show a clickable thumbnail within your post.
If you have an image on your computer that you'd like to put in your post, click the "Upload a File" button below the editor (between "Preview..." and "More Options..." This will allow you to upload the image. Once it is uploaded, select "Full Size" to show the full-sized image within your post or "Thumbnail" to show a clickable thumbnail within your post.