In the Civ IV Multiplayer Stories subforum. There is also a protected subforum where all visits are locked, if you are paranoid. Not looking into competitors stories is a matter of honor (and can lead to ban).
The link to create a new thread is at the top left, directly above the thread list.
An exhaustive list of BB-Codes can be found here.. Of note is the "attachent" section: Attach images (max. five) by editing your post in the enhanced mode ("Erweitert", next to "Antworten" below your normal reply box) and clicking on the paperclip button. After attaching them you can insert them by clicking on the "att2" link in the attachment list below your enhanced editing window.
Be advised that during editing attachments are only stored for a limited time. It is not advisable to attach first and write (long) texts later. One best practice is to write first and attach and insert last.
PM me for further questions.