BBCodes already in use:
[b][/b], [u][/u], [i][/i], [p][/p], [img][/img], [url][/url], [list][/list], [*][/*]
Bold, underline, italic, paragraph, image, link, list and list item respectively.
So for an image, you would use:
[img=http://www.link.com/to/file.jpg]
Which reminds me of something else I want to do: better BBcode processing and custom codes.