How to upload a file:
Press Start, Select File Browser, Select Most Downloaded, Select Screenshots, Press , Select Download, Back out to File Browser, Select Local Files, Select Screenshots, Press , Select Upload to File Share.
How to create a screenshot/video:
After playing a campaign level/firefight match/multiplayer game. The footage from what you have done will be saved in Theater. Go to Theater in the main menu, and choose your film. (Your most recent game you played will be already selected.) Start the film, and in the bottom right you will see:
Take Screenshot(For screenshots)
Record Clip(For video clips)
Choose one of these options and once you have taken the screenshot or video. It will give you the option to either:
Autoname and Save
Chose one of these options, and you have saved a screenshot or video.