4K HD Wallpaper: Steel Wool
The 1st 4K image from portfolio of Kamesh Vedula featured in gallery and from his courtesy is completely free under CC0 license. Download the photography if you like for use in your non-commercial (desktop backgrounds, homescreen phones) and commercial (apps, templates, blogs) 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)