Very easy- Edit your post, click the "preview post" button, right click in the image and select "copy shortcut", then click on the image icon and paste the information into the eidt box (I'm assuming you're using Windows & IE Explorer). Macs and other browswers should work in a similar manner. I copied your image from your post.
another way is to post the image on a web site (like Flickr.com) do the same thing. Flickr lets you copy the BB code for an image