There are actually many ways.
1. Click on "My Media" and you can add a photo from there that you already used in any other place.
2. Click on the little picture icon, and you can add a photo from another site if you have a link to it.
3. click on "More Reply Options" right bellow the reply box, if you are replying to a post, and that will give you another option to Attach Files. If you are starting a new topic, Attach Files is just bellow the box where you type your message.
4. You can start by uploading images to an album in the gallery and then either sharing a photo or the entire gallery by following step 2.
There is also information available here