There are a couple of ways. The first way is to attach them directly from your computer hard drive. When you hit either the "Post Reply" button just below the most recent post or the "Go Advanced" button below the quick reply box, you will get to a reply page. Among the options on the reply box is an icon that looks like a little paperclip. If you click on that, it will let you select files from your computer to attach to the post.
Or, you can do like many of us do and upload your picture to another site like Photobucket, Flickr, etc. Then, you can just copy and paste them into your post.