Click the reply button, type your message. Then under the white box click on the Attachments and other options sentence. Then use the browse button to select your file, repeat up to a maximum of 10 pics, then his the button that says post.
IF the photo's are small enough ie under 300kb in size than they will be added automatically to your post. If bigger than this you will need to use a program like PIXresizer to shrink them first.