there's two different ways you can do this, I'll explain the more user-friendly of the two. You must first go into posting mode:
1. Click the "Post Reply" button at the bottom left hand portion of the screen.
2. You will now be able to reply to a thread. I believe you must type a message at least 10 characters in length to post, so please post in the white message box something along the lines of "Picture for Wretched Ratchet"
3. Scroll down below the three buttons that say "Submit Reply", "Preview Post" and "Spell Check"
4. Under additional options, click on the button labeled "Manage Attachments"
5. A screen will pop up and you'll want to click on the top button labeled "Browse..."
6.You are now browsing through your computers files. Click on the file of the picture you would like to upload and press the "open" button.
7. You will now be back at the manage attachements screen and the file directory will be listed for your image in the bar next to the browse button you pressed.
8. Now press the "Upload" button on the bottom right.
9. The file will upload and then you will see the file directory listed below after it finished uploading.
10. Click the button labeled "Close this window"
11. You will now be back at the Reply to Thread screen. Click the button labeled "Submit Reply"
You reply with photo will now be posted.