You have to use an image uploading site.
http://www.tinypic.com is the best because you don't have to make an account.
http://www.photobucket.com is good because you can upload multiple pictures at once, but you have to make an account.
To post an image, you click the "post reply" button.
Then click the button underneath the
Italics button and on top of the
It will give you something that looks like {img]{/img] with different looking brackets
You simply upload your picture to one of those sites, click the
button and then paste the link which will look something like
http://i56.tinypic.com/2n8rdhv.jpg in between the two img brackets.
It's as easy as that!