4K HD Wallpaper: Miss with Donut
Thomas Kelley took this photography and from his courtesy can be saved free under CC0 license. Not quite a really cool image for a desktop background or homescreen phone, but I'm pretty sure that will be very useful to use in blog articles, web design projects, social media, content marketing...
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)