Windows 10 comes preloaded with various background themes you can always check them out. You can get more from Not sure if you have to pay them, haven't used their site.
Fair enough, I don't really mind of the pre-loaded wallpapers xD If I need to get a custom wallpaper, I just go to google images, paste them on paint and change my background from paint ¯\_(ツ)_/¯
Screenshot - f62ac89f7f0f955dc40b09ac78775196 - Gyazo Screenshot - b9186ea75d4e4e97e93b9953d084fc47 - Gyazo Screenshot - 3c1a44d27c7c6f45970315e9c0a325b1 - Gyazo Screen capture - fc4b9c401ec33f40c200189ee476e9fa - Gyazo
its 4 bucks that i dont have lol Edit: i didnt even buy windows so wallpaper engine is low on my list rn LOL
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I personally use the Hubble deep field as my desktop background. It's a 6400x6400 picture of tons of galaxies in deep space taken by the Hubble telescope. I can send you the full res file if you'd like (took a bit of poking around to find the full 6400x6400, most of the available downloads are downscaled).