4K HD Wallpaper: Natural Sky. Aurora. Northern Lights
From Yigit's portfolio and from his courtesy here's a 4K picture that looks insane and can be saved free for personal use as lockscreen phones and desktop backgrounds.
Widescreen Desktop / Macbook (Retina): 2880x1800 (fits on: 2560x1600, 1920x1200, 1440x900)
High Definition HD 1080p: 1920x1080 (fits on: 1600x900, 1366x768, 1280x720)
Samsung Galaxy Tab & iPad: 2048x2048 (compatible with almost all tablets)
HTC, Samsung Galaxy & iPhone: 1080x1920 (compatible with many smartphones)
Camera: PENTAX K-30 | Taken: March 1, 2014
Settings: Exposure time: 10 sec | ISO / Flash: 1600 / NO