The easy solution, for minimal use, would be just the IMG command.
[img]http://image_url[/img] is the format.
Rigth, except that most smilie sites don't have separate urls for each smilie. For example, see http://www.hostboard.com/ubb/smilies/smilies.html
. When you go to sites like that, you can usually drag or copy and paste those smilies onto email posts, but you can't do that onto a phpbb post, it seems.