Simple bash script to randomly set a `launching.jpg` runcommand image. Place in `/opt/retropie/configs/all`. More info at https://github.com/retropie/retropie-setup/wiki/runcommand#adding-custom-launching-images.

runcommand-onend.sh

#!/bin/bash
# 1.1
# (c) Jun 2020 Kelly Lawrence

# Set current directory to location of this bash script.
cd "$(dirname "$0")"

# Randomly rename all (*.jpg) image files in current directory.
for F in *.jpg ; do
  sudo mv $F $RANDOM.jpg
done

# Get one random (*.jpg) image file.
ImageFile=$( find ./ -maxdepth 1 -name "*.jpg" | shuf -n 1 )

# Rename selected (*.jpg) image file to `launching.jpg`.
sudo mv $ImageFile launching.jpg