4K HD Wallpaper: Shanghai City
Super urban landscape taken at night by Riku Lu and available from his courtesy completely free under CC0 license. I like this photography especially as desktop background, but it's also optimized for homescreen phones or to be used in blog articles, templates, apps...
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)