You now have the ability to upload background images hosted on
YouTube. To add a customized background image to your channel,  follow
the steps below:

1. Go to your Channel Design page: http://www.youtube.com/my_profile_theme

2. Select "Background Image."

3. Use the "Browse" button to find an image on your computer you would
like to use. Upload an image that will display as a background on your
channel (maximum 256K).

4. Decide whether you'd like your image to repeat across your channel
or appear only once. Choose "Yes" or "No" next to the "Repeat
Background Image" text.

5. Click the "Update Channel" button.

(NOTE: You can no longer use a URL linking to an image hosted by
another site for your background image. If you still have a URL in
your Channel Design settings, you will need to upload a new image
using the instructions above. Animated images are also no longer
allowed).

Thank you,

YouTube Timmy