AMOLED mnml Wallpapers is a free (and ad-free) cloud-based app for downloading a range of minimalist wallpapers.
You can opt for starkly simple dark designs, reducing power consumption by 5-15% and increasing battery life.
There are lighter and more colourful designs organised into categories like "Alien Hives", "Flower", "Geometric", "Grid" and more.
The basic wallpapers are in 3200 x 2560 resolution. They can be cropped before applying, or you can download them for use anywhere.
The app offers full Muezi live wallpaper support, giving you even more image options. There's also an XL version available which adds many more wallpapers up to 6400 x 5120 resolution for the best possible quality.
There are plenty of similar apps around, but AMOLED mnml Wallpapers is ad-free and easier to use than most.