You will need to get a photo from your camera to your computer.
Click "Post Reply" at the bottom end of thread
Scroll down the screen a little bit.
Find and click "Manage attachments"
You will get a pop up, click "browse" on the popup
Find the photo you put on your computer and click it.
You can write anything you would like in the post in the big text box, otherwise just press "submit reply" underneath it.