Treeko, you could either attach them to your post... make sure "I want to preview my post an/or attach file" is checked.... or you can use a free image hosting site like
http://www.photobucket.com (which has a nice, easy tutorial on how to upload files there). Once you've done that, select the picture and at the bottom, click on "generate code". Then copy the code for forums and paste it in your post.