Pictures say more than words -for teachers (and higher) who want to insert pictures in a forum, Moodle has made things quite easy. This procedure is exactly the same for other Moodle activities, by the way. Do note that students haven't got the possibility to add pictures on the fly -that is a safety measure, an attempt to reduce/prevent spamming. In their forum posts (and in other activities too) students can only link to pictures which are already online somewhere.