I'm sorry to say that your URL looks fine on my home system:
![Image](http://www.lepp.cornell.edu/~seb/celestia/hires-clouds2.jpeg)
System:
256MB 500MHz P3, WinXP Pro SP2
128MB GF4 Ti4200, Forceware v61.76
Celestia v1.3.2 final
Unfortunately, Chris had to hack the phpbb Forum software so that it'd even accept Cel://URLs, but they still can't be used in the embedded format, so you don't have to apologize.
![Sad :(](./images/smilies/icon_sad.gif)