When you hit the reply button (or post a new message) there is a link beneath the textfield called "additional Options" here one can attach certain file formats to the message.
However this is highly UNrecommended because when users post lots of attachments it takes up quite some space on the server where the forum is hosted.
The most preferable way to include images in a posting is to upload them elsewhere (e.g. flickr, Picasa or your own domain), and then include the image file in your post by copy and pasting the URL of your image between two code tags, like this:
Note that unlike in HTML the tags are in straight brackets [ and ] and not in < and >