• View the picture you want in the resolution which matches your display
• When image is visible in browser, right click and select Set as Background

MAC OS X

• View the picture you want in the resolution which matches your display
• Ctrl-click and select Download image to Disk
• Save image to desktop
• Go to Apple menu > System Preferences > Personal section and select Desktop
• On the Collection pulldown menu, go to Choose Folder and locate the appropriate picture.