If you aren't using the built-in Steam Workshop from Wallpaper Engine, you’ll need to download a video file (MP4 or WebM) manually:
: Offers a dedicated tag for Mikasa Ackerman with various 4K and HD options. Attack On Titan Mikasa Anime Live Desktop
(Paid): Available on Steam for approximately $4.99. It features the largest library of user-created Mikasa wallpapers, including 4K and interactive versions. If you aren't using the built-in Steam Workshop