You take the screenshots, and they'll appear on bunige.net in the "screenshots" section on your profile (if you linked your gamertag). You can save them to your computer from there.
When you save to your computer though, go to a site such as tinypic.com and upload a picture. Then copy & paste the forum link on your thread. 2 things before I go though, 1. Dont double post like that 2. This shouldnt be on the Halo Discussion