In the Ubuntu 12.04 community wallpapers post, I was telling you about some plans to provide packages for the wallpapers in each Ubuntu version. Well, as it turns out, this hasn't been postponed after all and an updated "ubuntu-wallpapers" package has landed in Ubuntu 12.04 Precise Pangolin which lets you install the community wallpapers from Ubuntu Karmic, Lucid, Maverick, Natty, Oneiric and obviously, Precise.
You can either get the wallpapers from a specific Ubuntu version by installing the "ubuntu-wallpapers-UBUNTU-VERSION" package (example: ubuntu-wallpapers-karmic), or all the wallpapers from Karmic to Precise, by using the following command:
sudo apt-get install ubuntu-wallpapers-karmic ubuntu-wallpapers-lucid ubuntu-wallpapers-maverick ubuntu-wallpapers-natty ubuntu-wallpapers-oneiric ubuntu-wallpapers-precise
If you're not using Ubuntu 12.04 and want all the community wallpapers from Ubuntu Karmic to Precise, download THIS archive.