Here is the full list of all 17 Wallpaper Engine achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Post a scene wallpaper to workshop
Use 100 different wallpapers from Workshop
Use 50 different wallpapers from Workshop
Use 25 different wallpapers from Workshop
Cause 1 fatal error OR trigger 128 warnings in the editor
Favorite 10 different wallpapers from Workshop
Use 10 different wallpapers from Workshop
Post a video wallpaper to workshop
Post a web wallpaper to workshop
Post an app wallpaper to workshop
Use a 2D scene template
Use a 3D scene template
Post a Steam screenshot with a workshop wallpaper open
30 lives
Favorite 20 different wallpapers from Workshop
Favorite 30 different wallpapers from Workshop
Favorite 50 different wallpapers from Workshop