Take your screen shots just like any other time. Then go to your bungie.net profile to view them. Then save them to your computer and upload them to a website like Photobucket. Then use those URLs to embed your pics in the thread. For more info, read here: http://www.forgehub.com/forum/halo-forge-maps/18644-embed-screenshots-your-map-thread.html And here: ForgeHub Halo Forums - Announcements in Forum : Halo Forge Maps
?? have you never posted a map before? you can just use the theatre mode in halo 3 to take screenshots, subscribe to bungie and the screenshots automatically upload to bungie. then download them from bungie to your computer and then upload them to photobucket and use the code thats on photobucket to post the images on the forum. hoped that helped, if you need a bit more help pm me