Irvue is an app from Mac App Store that does exactly this. Irvue automatically sets your wallpaper from Unsplash, a free, high-quality stock photo website with photos from professional and amateur photographers around the world.
One of Irvue's main feature is that you can easily set update intervals, i.e. how often you want your wallpaper to change, from every 30 minutes, every few hours, every 24 hours, every few days, up to every month. I personally like to have it change my wallpaper every 24 hours, and if you happen not to like the wallpaper that Irvue changed for you, you can always set your current wallpaper manually.
Also, you do not have to download any of the wallpapers as Irvue sets your wallpaper directly from Unsplash, so there's one less thing to organize, but you're certainly free to do so if you happen to have any favourites!
Here are some wallpapers that Irvue has set for my desktop: