There's 2 ways. When you start or reply to a post, under the subject is a spot that says attachment, you can upload it from your computer by clicking browse and then finding the picture on your computer.
Secondly, you can use tinypic.com/ Upload the picture, select image, select "Website/Email" for size, then click "Upload Now". Copy the Image code for Forums & Message boards which looks like this: i46.tinypic.com/numbers .jpg[ /IMG] - Copy and paste the link into the reply box and click post reply.