If you want to insert pictures directly into forum posts and replies, you should change appropriate datatype to <i>XML block</i> (by default forum classes use <i>text block</i> datatypes for message text). After this user will be able to insert picture as a embedded object into his/hers posts. Of course, you must also define appropriate policies for creation of image objects.
Alternatively, if you do not want to give your users all the functionality XML block attributes bring, you could add an additional, image attribute to forum post and reply classes, by which users could upload pictures attached to their posts.
