Xbox – Daily Challenges Post Times

According to this thread on GameFAQs, you need to play a new game on hard.

You can take screenshots of saved films in the Theater.

Every time you finish a game, it is saved (temporarily) to your hard drive (I think that temporary films are deleted when you exit the game, but I'm not sure). In the Pause menu, there is an option to view local files, and in there is a section for your temporary films. Here you can either delete them, or save them to your hard drive permanently.

You can replay these films in the Theater, which is accessible from the main menu. You can load any locally saved film (including the temporary ones) here, and watch the entire match again. While watching, you can either create a film clip, or pause the action and take a screenshot.

When you're in a replay, you can press start to see the various controls available to you.