I'm sure you figured this out but if you just put the following in front of, and at the end the URL to your picture it will show in the post.
So, if this is your URL:
PHP Code:
http://farm4.static.flickr.com/3130/2847000329_c77854bb02_o.jpg
To get it to show up in your post, you'd do this:
PHP Code:
[img]http://farm4.static.flickr.com/3130/2847000329_c77854bb02_o.jpg[/img]
Voila!