Where can I find all the wallpapers ever included?

There are mainly two ways to find wallpapers included in all Ubuntu official releases: Ubuntu repositories and Ubuntu Package Search.

From Ubuntu repositories

Ubuntu has four repository components, which are all enabled by default. The main component will contain wallpapers for current Ubuntu release and universe component will contain wallpapers for previous Ubuntu releases and other flavours.

Open a terminal and run the following command.

apt-cache search ubuntu wallpapers | sort -n

This will return a local search result: use "ubuntu wallpapers" as keywords and find package that matches with the name, then sort the result by string numerical value.

blubuntu-wallpapers - Blubuntu look - Wallpapers
edgy-community-wallpapers - Edgy Community Wallpapers
edgy-wallpapers - Edgy Wallpapers
edubuntu-artwork - edubuntu themes and artwork
edubuntu-wallpapers - wallpapers included in edubuntu
feisty-wallpapers - Feisty Wallpapers
gutsy-wallpapers - Gutsy Wallpapers
ubuntu-gnome-wallpapers-trusty - Ubuntu GNOME 14.04 Wallpapers
ubuntu-gnome-wallpapers - Ubuntu GNOME Wallpapers
ubuntukylin-wallpapers-saucy - Ubuntu Kylin 13.10 Wallpapers
ubuntukylin-wallpapers-trusty - Ubuntu Kylin 14.04 Wallpapers
ubuntukylin-wallpapers - Ubuntu Kylin Wallpapers
ubuntustudio-wallpapers - Ubuntu Studio - Wallpapers
ubuntu-wallpapers-karmic - Ubuntu 9.10 Wallpapers
ubuntu-wallpapers-lucid - Ubuntu 10.04 Wallpapers
ubuntu-wallpapers-maverick - Ubuntu 10.10 Wallpapers
ubuntu-wallpapers-natty - Ubuntu 11.04 Wallpapers
ubuntu-wallpapers-oneiric - Ubuntu 11.10 Wallpapers
ubuntu-wallpapers-precise - Ubuntu 12.04 Wallpapers
ubuntu-wallpapers-quantal - Ubuntu 12.10 Wallpapers
ubuntu-wallpapers-raring - Ubuntu 13.04 Wallpapers
ubuntu-wallpapers-saucy - Ubuntu 13.10 Wallpapers
ubuntu-wallpapers-trusty - Ubuntu 14.04 Wallpapers
ubuntu-wallpapers - Ubuntu Wallpapers
xubuntu-community-wallpapers - Xubuntu community wallpapers
xubuntu-wallpapers - Xubuntu wallpapers

The local search result will vary by releases: 14.04 release will show above output; 16.04 release will also include wallpapers from its current and earlier releases, which are 16.04, 15.10, and 15.04.

From Ubuntu Packages Search

For users running an older release but still want to get wallpapers for the newer releases, open a web browser and visit Ubuntu Package Search.

This site provides you with information about all the packages available in the Ubuntu Package archive.

Scroll down until you reach "Search package directories". The suggested search options are:

• Keyword: ubuntu wallpapers
• Search on: [x] Package name only
• Distribution: any
• Section: any

Click Search to see the result.

Package blubuntu-wallpapers

* trusty (14.04LTS) (x11): Blubuntu look - Wallpapers [universe]
    0.3.1: all
* xenial (16.04LTS) (x11): Blubuntu look - Wallpapers [universe]
    0.3.1: all
* bionic (18.04LTS) (x11): Blubuntu look - Wallpapers [universe]
    0.3.1: all
* cosmic (18.10) (x11): Blubuntu look - Wallpapers [universe]
    0.3.1: all
* disco (x11): Blubuntu look - Wallpapers [universe]
    0.3.1: all

Package edubuntu-wallpapers

* trusty (14.04LTS) (gnome): wallpapers included in edubuntu [universe]
    14.03.1: all
* xenial (16.04LTS) (gnome): wallpapers included in edubuntu [universe]
    15.12.1: all
* bionic (18.04LTS) (gnome): wallpapers included in edubuntu [universe]
    15.12.1: all
* cosmic (18.10) (gnome): wallpapers included in edubuntu [universe]
    15.12.1: all
* disco (gnome): wallpapers included in edubuntu [universe]
    15.12.1: all

...

The result will include packages with name that matches with "ubuntu wallpapers" keywords. It is also possible for users to download the DEB packages from the links in the search result and install to any release in use, since there are no dependencies for installing the wallpapers.

Search result discrepancy

Wallpapers from some releases are missing from the search result

Ubuntu Package Search will show all available packages regardless of the release. In comparison, the local search result from Ubuntu repositories depends on the release that is currently used.

For example, 14.04 release does not show ubuntu-mate-wallpapers-* in the Ubuntu repositories because Ubuntu MATE became an official flavour from 15.04 release. With Ubuntu Package Search, you can find such missing packages easily.

Searching with "ubuntu wallpaper" does not show certain packages

The suggested search option will show most relevant results. The keywords will match packages that contain wallpapers for all official flavours, given that the names were derived from and contain "ubuntu" itself: Ubuntu, Lubuntu, Kubuntu, Xubuntu, Ubuntu MATE etc.

Therefore, some wallpapers are missing from the search result because of different package naming, such as wallpapers for Plasma Workspace and Shimmer Project.

This answer is applicable to all supported Ubuntu releases at any time.

Found the following site which seems to have all of them (from 4.10 to 10.10 only).

http://ubuntu.ecchi.ca/wallpapers/