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.
Use 25 different wallpapers from Workshop
Use 50 different wallpapers from Workshop
Use 100 different wallpapers from Workshop
Favorite 20 different wallpapers from Workshop
Favorite 30 different wallpapers from Workshop
Favorite 50 different wallpapers from Workshop
Post a Steam screenshot with a workshop wallpaper open
30 lives
Cause 1 fatal error OR trigger 128 warnings in the editor
Post an app wallpaper to workshop
Favorite 10 different wallpapers from Workshop
Use 10 different wallpapers from Workshop
Post a web wallpaper to workshop
Use a 3D scene template
Post a video wallpaper to workshop
Post a scene wallpaper to workshop
Use a 2D scene template