Using Reg.exe from the Windows 2000 Support Tools, or Reg.exe that is built into Windows XP and later, and SidToName.exe freeware, I have scripted WhatWallPaper.bat to return the computer name, user name, and wallpaper that is configured in the Wallpaper Value Name at "\\ComputerName\HKU\<SID>\Control Panel\Desktop" for each user of a computer.

The syntax for using WhatWallPaper.bat is:

WhatWallPaper \[ComputerName1 ComputerName2 ... ComputerNameN

Where each ComputerNameX is optional.

WhatWallPaper.bat contains:

@echo off
setlocal ENABLEDELAYEDEXPANSION
if \{%1\}
\{\} set comp=\\%computername%\&set cmp=%computername%\&goto doit
:again
if \{%1\}
\{\} endlocal&goto :EOF
set comp=%1
set cmp=%comp:\=%
set comp=\\%cmp%shift
:doit
for /f "Tokens=*" %%a in ('reg query "%comp%HKU"^|find "\"') do (
 set user=%%a
 set user=!user:HKEY_USERS=HKU!
 set sid=!user:HKU\=!
 set wp=NONE
 set usr=NONE
 call :getwp>nul 2>&1
 if "!wp!" NEQ "NONE" @echo "%cmp%","!usr!","!wp!"
)
goto again
:getwp
for /f "Tokens=2*" %%b in ('reg query "%comp%!user!\Control Panel\Desktop" /V Wallpaper^|find "REG_SZ"^|find /v /i "(None)"') do (
  set wp=%%c
)
if \{%wp%\} NEQ \{\} set wp=%wp:"=%
if "%wp%" EQU "" set wp=NONE&goto :EOF
if "%wp%" EQU "NONE" goto :EOF
for /f "Tokens=*" %%x in ('call SIDtoName !sid! %comp%') do (
 set usr=%%x
)