To apply a wallpaper in dwm and you have lightdm, you can use this solution

Install feh

$ sudo pacman -Syu feh

Create a script that will run feh

$ touch ~/run-dwm
$ chmod 755 ~/run-dwm

Update script

Use your preferred editor to edit the script

$ vim ~/run-dwm

Add the following lines. Change the path to your desired wallpaper

#!/bin/bash

feh --bg-fill /home/user/Pictures/wallpaper/003.jpeg
dwm

Update .desktop file

$ sudo vim /usr/share/xsessions/dwm.desktop

Change the value to the path of the created script

[Desktop Entry]
Encoding=UTF-8
Name=Dwm
Comment=dynamic window manager
Exec=/home/user/run-dwm
Icon=dwm
Type=XSession

Restart your dwm session

Reference

https://bbs.archlinux.org/viewtopic.php?id=191898