Right beside the "submit" button is an "Attach Files" button. Click it. Then select "Browse", which will take you to pics on your computer. Click on your picture. Then submit.
Hint: I try to go into my pictures first, and make a copy of the picture I want to post, then I edit it with paint. I choose the sketch/skew option and reduce the pic to 25% of the original size. That way it doesn't take forever to upload and upload time to viewers is also reduced. I have dial-up!