How do I extract a background from a Windows 7 Theme file?

In the Personalization Control Panel save the theme for sharing as a .themepack file - right click on the current theme and choose "Save theme for sharing". Then open the saved themepack file with 7Zip or similar and extract the images you want.

You can find those wallpapers on your C drive after you install a theme

Navigate to

C:\Users\(your user name)\AppData\Local\Microsoft\Windows\Themes

with 7-Zip,

1) right click the file, select Open archive
2) open DesktopBackground folder
3) select the wallpaper you want or select all by mouse pointer, then click on extract option

now check your extracted location