A successful visual identity has to be consistent, recognizable and powerful.
The Official Brand Guideline includes all the artwork you could need for your site. It also includes a set of tips designed to help all of us create a professional, powerful, precise and consistent brand identity.
Please take the time to understand how to apply it correctly.
You can find more logos, patterns and artwork for your project at here.
If that isn't enough go to the Artwork Team or send a message to: <ubuntu-art AT lists DOT launchpad DOT com>