4K HD Wallpaper: Alfa Romeo 8C Competizione
The red beauty can be downloaded under CC By-Sa license from courtesy of Tom Wolf. Here's optimized for fit on tablets and desktop backgrounds.
Widescreen Desktop / Macbook: 2560x1600 (fits on: 1920x1200, 1680x1050, 1440x900)
High Definition HD 1080p: 1920x1080 (fits on: 1600x900, 1366x768, 1280x720)
Samsung Galaxy Tab & iPad: 2048x1536 (compatible with almost all tablets)