You can't just copy and paste off of Bungie.net. You have to embed the picture. You can find a tutorial here: http://www.forgehub.com/forum/halo-forge-maps/18644-embed-screenshots-your-map-thread.html
So, you dont have to click a link here is a breif way to help you with this. Question for you: Do you have photobucket? If you do this will be easy From bungie you have to save the pictures on to your computer ( preferably in one folder), then go to your photobucket acount if you have one. Upload the folders, or use the bulk uploader and upload the whole folder. Once that is done you need to paste the url of the pictures in the middle of this if you want me to do it for you I can.