The Ubuntu Wallpaper Flickr Group for Ubuntu 12.04 Precise Pangolin is open for submissions! You can go here to see whats been uploaded and to upload your own...
We have added submissions for most of the Ubuntu releases in the past few years. You can see what we submitted for 11.10 here:
If you are a photographer or a wiz with graphics, we hope you join in and contribute to Ubuntu. this is an easy way to help out the greatest linux distro! UBUNTU!