To embed pictures in a map post you would go to a image hosting site such as: holescreenshots.net or to imageshack.us for non-halo things. Haloscreenshots you will need to recover your gamertag and follow the directions on the page.
Well, to make a new thread you would go to the appropriate section of the forum let's say the 'Halo Forge Maps' Scroll down to the very bottom of the page and click on 'New thread', Which that should bring you too a new page with a box where you would put in the text/pictures. After you are done with the new thread click on 'Post new thread'. To post a quick reply let's call the 1,000,000 million post party countdown. Scroll down to the bottom and you will see a box that you can type a short little reply in or a long reply (Your choice). When you are done you would click on the button that say's 'Post quick reply'. That should be all you need to know to post on our forums.