Pretty easy.
You first need to "host" the photo somewhere. Either here (which is very limited and not worth the trouble, imo) or on a free image hosting site. I use picasa and photobucket. Picasa is easiest - runs on your comp and uploads with one click.
Then, your hosting site will give you the correct image link, and you simply paste that into the post.
If it seems complicated, its not. Just play with it, you'll get it.