To copy your Background that you have picked, right click on the large
background picture that you chose. Click on the "Save picture as ..."
Then save it in your My Picture directory (so you can find it, see below).
Now that you have your Background saved, go to YouTube.com to your Website
Then click on the Name of your Website at YouTube to make the dropbox
come down. Then click on "My Channel" to go to your Channel.
Now that you are at your Channel, look for the "Channel Settings" and Click on it.
Once you have clicked on the "Channel Settings" you will be able to chose
your picture to upload to your channel as the Background. Click on the
"Choose File" button under Background.
Now go to the directory where you saved it, as your My Picture directory, and
click on the Background, then click the "Open" button.
When it is finished uploading you can see what it will look like, but to
make it permanent you need to Click on the "Done Editing" button at the
top (see above).