4K HD Wallpaper: Pure Fire
Really hot 4K image this one taken by the guys from the Skitter project. From their courtesy the photo is completely free under CC0 license and can be used in non-commercial (desktop backgrounds, homescreen phones) and commercial (blog, apps, content marketing, social media...) projects.
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 many tablets)
HTC, Samsung Galaxy & iPhone: 1080x1920 (compatible with many phones)