I recommend IMGUR. You don't need an account or captcha. Either upload it from your machine or copy and paste the URL like the poster above me said. In your post it would look like
Code:
[IMG]http://i.imgur.com/s9Frc.png[/IMG]
Using it directly from a URL rather than uploading it yourself is called hotlinking and it's not always a good idea (some sites have anti-hotlinking images)