4K HD Wallpaper: Polar Bear with His "Ice Cream" :)
The 4K picture was made at the Rotterdam Zoo and it's available under CC By-Sa license from Sander's courtesy. Here's optimized for fit on desktop backgrounds and homescreen phones.
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)
HTC, Samsung Galaxy & iPhone: 1080x1920 (compatible with many smartphones)