Each Ubuntu release ships with 15-20 beautiful wallpapers created by the community. For every Ubuntu version starting with Karmic (9.10), there's a package that contains all these community wallpapers, which you can install in newer Ubuntu releases.
By using the instructions below, you can install all the Ubuntu community wallpapers from Ubuntu 9.10 Karmic Koala to Ubuntu 13.10 Saucy Salamander (138 wallpapers!), in the latest Ubuntu 14.04 Trusty Tahr.
If you don't use Ubuntu or you're using an older release, there's a download link at the end of the article which allows you to download all the wallpapers from Ubuntu Karmic Koala (9.10) to Trusty Tahr (14.04).
Here are a few of the community wallpapers used in previous Ubuntu versions (the images below are for presentation purposes and have been scaled down, don't use them as wallpapers!):
... and many others.
Install the Ubuntu community wallpapers from Karmic to Saucy in Ubuntu 14.04 Trusty Tahr
By using the command below in Ubuntu 14.04, you'll get 138 wallpapers (around 35-40 mb) that were used in previous Ubuntu releases:
sudo apt-get install ubuntu-wallpapers-karmic ubuntu-wallpapers-lucid ubuntu-wallpapers-maverick ubuntu-wallpapers-natty ubuntu-wallpapers-oneiric ubuntu-wallpapers-precise ubuntu-wallpapers-quantal ubuntu-wallpapers-raring ubuntu-wallpapers-saucy
(the wallpapers are installed under /usr/share/backgrounds/ and they should show up automatically under System Settings > Appearance)
The command above doesn't include "ubuntu-wallpapers-trusty" because the community wallpapers for Trusty are already available by default in Ubuntu 14.04, obviously.
If you don't use Ubuntu or you're using an older release, download all the Ubuntu community wallpapers from Karmic to Trusty, from HERE.