Go to Gallery at the main menu, choose a background, take a picture, upload to Facebook, rinse and repeat, pretty simple stuff.
If you don't want your Facebook flooding with these pics then on Facebook go to 'Settings' and then on the left hand side choose 'Apps'. Find 'Xbox Advertising Originals', click edit, and then change the 'Visibility of app' to 'Only me'. Now you can upload the pics without them appearing at all on everyone's Facebook.
After your achievements you should also change the settings of the app so it doesn't post on your behalf, and delete the photo's it has uploaded too.
See comments if you have difficulty uploading.