In order to post a picture, it must be hosted somewhere on the web. If it's only on your computer, no one else will be able to see it.
Good photo hosts are: Image Shack, Tiny Pic, and Photobucket. They are all free. Once you have uploaded a picture to their website, you want to copy/paste the URL for your picture into a post. Look underneath the picture for different URLs. For this website, you want the one with the IMG tags at the beginning and end.