wallpaper

I'm not sure if it's the same on older systems, but on ICS it appears to save directly to /data/data/com.android.settings/files/wallpaper. I set mine from the browser, pulled that file, then changed it to a .png extension and viola - it was the image I had set.

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).