Wrap i3lock to randomly select an image from a folder and automatically scale it to fit your monitor resolution.

i3lock-wrapper.sh

#!/bin/bash
random_wallpaper="$(find $1 -maxdepth 1 -name *.png | shuf | head -1)"
actual_size="$(identify -format %wx%h $random_wallpaper)"
target_size=$(xrandr | head -n 1 | awk -e '{OFS=""; gsub("[ ,]+","",$10); print $8,"x",$10;}')

if [[ "$actual_size" != "$target_size" ]]; then
	convert "$random_wallpaper" -background black -gravity center -scale $target_size -extent $target_size png:- | i3lock -i /proc/self/fd/0
else
	i3lock -t -i "$random_wallpaper"
fi