Hi there. I believe that currently the only way for users to be able to embed images or use HTML in their posts is to be given edit permissions to the page the forum is on, which effectively makes them a forum administrator. I think the ability to embed external images is a good feature request. Perhaps externally linked images could be allowed to be used for users who are marked as trusted? I know that some other forums have their own special tags to allow for a subset of HTML for things like this ([img][/img]). That way, you don't have to allow the user to use HTML (the forum software translates before page rendering).
Hopefully Joe will chime in on this subject when he can.