Make sure you're connected to XBL when taking screenshots. If you're not connected, then obviously, they won't upload, and they won't do it automatically the next time you sign in. Unfortunately, there's no manual way to do this. I guess the only solution would be to save those films you used, and retake the screens when you're on XBL.
Or, if you want the screens for a desktop background or something like that, upload the screens to your file share, and then view your file share on B.net. You won't be able to get those into your Service Record gallery, but at least you can use them as desktops.
[Edited on 11.03.2007 5:00 AM PDT]