Remember not to keep an image up all the time which can use burn-in. The classic is having a windows desktop as those icons remain at the same location all the time.
Wallpaper Cave has a collection of 4K OLED wallpapers designed for PC screens. The reason why some of these wallpapers have extensive black areas is not just to show off your OLED dark contrast levels, but to allow you to use the image at night too.
https://wallpapercave.com/4k-oled-pc-wallpapersWallpaperAccess has high-contrast and vibrant OLED 4K backgrounds.
https://wallpaperaccess.com/oled-4kHDQ Walls has mountains and sunset OLED 4K wallpaper.
https://hdqwalls.com/wallpaper/3840x2160/mountains-and-blue-sunset-oled-4k 
