I'm very late in seeing this, but I noticed this thread today, which points people to an xbox.com article detailing the usage rights for community people who distribute MS owned game 'content'. That pretty much includes any person or site that distributes images and/or video of material (games) owned by MS. Interesting stuff, and some good comments (along with some stupid ones) in this thread.

EDIT: I realized that I'm not just late, I'm VERY late on this issue, but I'll leave this up here for those of you (like me) who somehow managed to miss it. :)