Where can I find hidden wallpapers on Nexus 7?
I have Nexus 7 with stock Jelly Bean 4.2. Wallpapers are stored in /system/app/Launcher2.apk/res/drawable-sw600dp-nodpi as wallpaper_01.jpg to wallpaper_14.jpg but your picture is not here.

I checked wallpapers in version 4.1.1 online here:
https://bitbucket.org/neuralassembly/jcrom_packages_apps_launcher2/src/bd231599ae9f/res/drawable-sw600dp-nodpi

And your picture is not here too. What Android version do you have?