Guidelines and HOWTOs/Change the default wallpaper

Copy the old wallpaper from the breeze repo to the plasma-workspace-wallpapers repo. Don't forget to copy the whole Next folder, rename it appropriately, and also update the CMakeLists.txt file!
Generate multiple sizes/aspect ratios for new wallpaper with [breeze repo]/wallpapers/generate_wallpaper_sizes.py.
1. If the Mobile versions look ugly, manually make better-looking versions by cropping, rotating, etc.
Replace existing wallpaper images in [breeze repo]/wallpapers/Next/contents with new one. Don't forget screenshot.png!
Edit [breeze repo]/wallpapers/Next/metadata.desktop to reflect the new name, and include the name and email address for the author
Compile breeze
Create a new user account named "Konqi", log into it, switch the language to English
Run kscreenlocker_greet --testing and take a screenshot with no window decorations
Open Dolphin, the "About KDE" window, and Kickoff. Take a full-screen screenshot
Switch the global theme to Breeze Dark to dark and do it again
Switch the global theme to Breeze Twilight and do it again
Log back into your original user account and copy the screenshots into an accessible place.
Set your login screen to display the new wallpaper
Run sddm-greeter --test-mode --theme ~/kde/usr/share/sddm/themes/breeze/ and take a screenshot without window decorations
Replace the Breeze, Breeze Dark, and Breeze Twilight Global Theme preview images in plasma-workspace with the new ones you took in the Konqi account
Replace the lockscreen preview image in plasma-workspace with the new one you took in the Konqi account
Replace the SDDM preview image in plasma-workspace with the new one you just took
git push all your changes

This is a very fiddly, time-consuming, and complicated process. https://phabricator.kde.org/T13629 tracks improving it.