after a certain number of posts a big red button will appear at the bottom of the screen, between "POST REPLY" and "MORE OPTIONS"
however in the meantime you can use the button beside the
yface in the top row of your reply.
you will need to put your photo onto a free image hosting service such as imgur.com or photobucket.com (even facebook will do) and use the URL of the photo when you come to put your case here.