This is a very old topic, but I wanted to throw my script in so you can edit it to your liking. I don't have an Adafruit display, so I had to do some tinkering but you've all done the heavy lifting. Here are the minor edits for using a 3.5" display with fbi. Make sure you have the system.jpg images in the correct system rom folders.:
# /opt/retropie/configs/all/runcommand-onstart.sh
# get the full path filename of the ROM
rom=$3
# rom_bn receives $rom excluding everything from the first char to the last slash '/'
rom_bn="${rom##*/}"
# rom_bn receives $rom_bn excluding everything from the last char to the first dot '.'
rom_bn="${rom_bn%.*}"
# get the system name
system=$1
# set the image file to the first result matching the ROM name - accounts for various possible file extensions
img="$(find "/opt/retropie/configs/all/emulationstation/downloaded_images/${system}" -type f -name "${rom_bn}-image.*" | head -1)"
# check to see if there was a file found (length of the file name not zero), if not, use an image based on system
if [[ -z "${img}" ]];
then
img="/opt/retropie/configs/all/emulationstation/downloaded_images/$system/$system.jpg"
fi
# run the python script to display the image
sudo fbi -T 2 -d /dev/fb1 -noverbose -a "$img"
