WallpaperSafari and WallpaperCave are excellent for curated collections of "best" and trending images.
Zedge offers a massive library of user-uploaded content with a specific category for robot backgrounds. 1920x1080 Robot Wallpapers | Best Wallpapers
A popular niche includes small, "chibi" style robots or robots in unexpected natural settings, like holding a flower or wandering through a forest. 1920x1080 Robot Wallpapers | Best Wallpapers
“They have a very SOMA-y feeling, which makes them even better in my eyes. Kudos!” Reddit · r/MrRobot · 9 years ago 1920x1080 Robot Wallpapers | Best Wallpapers