4K HD Wallpaper: Samsung NX300
Hans Braxmeier is the photographer behind this image and from his courtesy can be downloaded free under CC0 license. Of course the image is optimized here for fit in desktop backgrounds and homescreen phones, but also can be used under the CC0 license terms in other projects that you might have.
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)