Unsplash has been my favourite website to get free premium stock photo for website prototyping. Unlike the standard boring stock photos, unsplash's photos have more cool vibe in it which significantly boosts my design.
Besides of web design, I use it for my wallpaper too. The good news is - they have release Unplash Wallpapers. You can access to the popular wallpaper collections from the menu bar.
Unfortunately, Unsplash Wallpapers is only available for mac OS.