Dynamically produced images like you have will work with Web Wiz Rich Text Editor, see demo below:-
http://www.webwiz.net/webwizrichtexteditor/demo/However, Web Wiz Forums works differently and when a post is submitted it is sent through various security filers to remove any content that could be harmful to hack the forum, website, or members accounts.
The security filters in web wiz forums strip out dynamic images. If they did not I could very easily write in a few minutes a fake dynamic image that I could post that could get your login details or trick you in to submitting fake information under your account.
Edited by WebWiz-Bruce - 03 June 2010 at 1:43pm