Ubuntu 18.10 BETA が出たらしいのだけど手元のマシンには壁紙が降りてこない
budgie-wallpapers-cosmic てのを見つけたけど違うぽい

$ lsb_release -d
Description: Ubuntu Cosmic Cuttlefish (development branch)
$ apt-cache search ubuntu-wallpapers
ubuntu-wallpapers - Ubuntu Wallpapers
ubuntu-wallpapers-bionic - Ubuntu 18.04 Wallpapers
blubuntu-wallpapers - Blubuntu look - Wallpapers
edubuntu-wallpapers - wallpapers included in edubuntu
kubuntu-wallpapers-bionic - Selection of classic KDE wallpapers for Bionic Beaver
ubuntu-wallpapers-artful - Ubuntu 17.10 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-utopic - Ubuntu 14.10 Wallpapers
ubuntu-wallpapers-vivid - Ubuntu 15.04 Wallpapers
ubuntu-wallpapers-wily - Ubuntu 15.10 Wallpapers
ubuntu-wallpapers-xenial - Ubuntu 16.04 Wallpapers
ubuntu-wallpapers-yakkety - Ubuntu 16.10 Wallpapers
ubuntu-wallpapers-zesty - Ubuntu 17.04 Wallpapers
xubuntu-wallpapers - Xubuntu wallpapers
$ apt-cache search wallpaper cosmic
budgie-wallpapers-cosmic - Ubuntu Budgie 18.10 Wallpapers
$ sudo apt install budgie-wallpapers-cosmic
$ dpkg -L budgie-wallpapers-cosmic|grep \.jpg$
/usr/share/backgrounds/budgie/bird-3158784cosmic.jpg
/usr/share/backgrounds/budgie/boats-3051610cosmic.jpg
/usr/share/backgrounds/budgie/coffee-802057cosmic.jpg
/usr/share/backgrounds/budgie/fish-3062034cosmic.jpg
/usr/share/backgrounds/budgie/gift-548290cosmic.jpg
/usr/share/backgrounds/budgie/green-tea-2356770cosmic.jpg
/usr/share/backgrounds/budgie/ice-cubes-3506781cosmic.jpg
/usr/share/backgrounds/budgie/knowledge-1052010cosmic.jpg
/usr/share/backgrounds/budgie/mattress-2489615cosmic.jpg
/usr/share/backgrounds/budgie/message-in-a-bottle-3437294cosmic.jpg
/usr/share/backgrounds/budgie/skyscraper-3094696cosmic.jpg
$ xdg-open /usr/share/backgrounds/budgie/ https://gnusocial.matoken.org/attachment/159196