4K HD Wallpaper: Water Drops
This macro free picture was taken by the same talented photographer - Olia Gozha - as previous photo and of course it's free under CC0 license. Personally, this type of image I use for my homescreen phone, but can be saved for desktop backgrounds, templates, blogs, 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)