Here's my situation:
I had an image from a website which I set as my background inside the browser. I am wondering now where I can find the image in the file system, because I can't find it online anymore. I am rooted.
I'm not sure if it's the same on older systems, but on ICS it appears to save directly to
It also appears that this file simply gets overwritten when you change your wallpaper, so I don't see a way to recover one once it's changed unless you have a copy of the image somewhere else.
This also appears to be where it's saved on CyanogenMod 7, so it's possible that this is the "standard" location (certainly a good place to check, at least).
I was annoyed by the exact same problem, so I have programmed an app that automatically saves your wallpapers and your live wallpapers too. It makes it easy to revert to a previous wallpaper. It is called Wallpaper Saver and it's free in the Play Store. (It does not require root access.)