@ossobuco,
It should look something like this:
Code:[img]http://www.newyorker.com/images/2012/05/07/p465/120507_contest_p465.jpg[/img]
1. Right-click the image, select "copy image location"
2. Start your new post
3. click the Img button (if you don't have an Img button, click the green line that says "Open BBCode Editor"
4. Past the URL between the image tags.
5. Type your caption below the line.
6. Submit your post.