@echo off&setlocal enabledelayedexpansion
for /f "delims=" %%i in (D:\Downloads_save\wallpaperlist.txt) do (
    set /a n+=1
    set _!n!=%%i
)
set /a lines=%random%%%%n%+1
"D:\Program Files\IrfanView\i_view32.exe" !_%lines%! /wall=2 /killmesoftly