Google's Wallpapers app is pretty excellent, providing an ample selection of high-quality backdrops for your smartphone, tablet, or Play Store-equipped Chomrebook. The app hit the Play Store just a couple years ago, and it's been tearing it up ever since, having already passed 100 million installs.
Like is often the case with these things, not all of those 100 million are intentional installations; the app comes pre-loaded on some phones, like Pixel devices (although those aren't exactly setting the sales charts on fire). It's available on any device running Android 4.1 or higher, though, so if you don't have it already, give it a look. You're sure to find something you like.