I was looking for a good source of random wallpapers to use on the lock screen, and it turns out that Unsplash is great.
This should work an any wlroots-based WM:
#!/bin/bash
WP_FILE=$HOME/.config/wallpaper.jpg
wget -q -O $WP_FILE https://source.unsplash.com/1920x1080/?night,ocean,space,nature
if [[ -s "${WP_FILE}" ]]; then
swaylock --image $WP_FILE
else
swaylock -f -c 000000
fi