Very rough photobooth script using gPhoto2, ImageMagick, and CUPS.

photobooth.sh

#! /usr/bin/env bash

# Extremely rough photobooth bash script
# It depends on the following:
# - gPhoto2: for camera control through usb
# - ImageMagick: photo-manipulation suite
# - CUPS: printer driver with command-line printing utilities
# - beep: for the countdown sound

save_dir=./processed
prefix=Xmas2011
overlays=(./overlays/*.png)
current_overlay=`cat ./overlays/current`
print=true
beeper=true

# Start infinite loop. 
if [ "$1" = start ]; then
    while true; do
        read -sn 1 -p "Press any key to start shooting. Ctrl-C to quit."
        # initialize shot count
        echo "0" > ./count
        gphoto2 -F 3 -I 3 --capture-image-and-download --force-overwrite --hook-script $0
    done
fi

# Checks if save directory exists, if not create it
! [ -d $save_dir ] && mkdir "$save_dir"

function process() {
    # Get the last number of the last shot
    all_files=($save_dir/*.jpg)
    last_file_num=`echo ${all_files[-1]:(-7):3} | sed 's/^[0]*//'`
    if [ -e ${save_dir}/${prefix}_001.jpg ]; then
        last_file_num=`printf "%03d" $((last_file_num+1))`
    else
        last_file_num=`printf "%03d" 1`
    fi

    # Process shots
    mkdir .tmp
    convert $4 -sigmoidal-contrast 3,30% -resize 1077x721 -rotate -7 .tmp/1.png
    convert $3 -sigmoidal-contrast 3,30% -resize 300x201 .tmp/2.png
    convert -sigmoidal-contrast 3,30% -resize 300x201 -rotate -12 -background transparent $2 .tmp/3.png
    convert -size 1800x1200 xc:white \
        .tmp/1.png -geometry +129+232 -composite \
        .tmp/2.png -geometry +156+117 -composite \
        .tmp/3.png -geometry +492+60 -composite \
        $1 -composite ${save_dir}/${prefix}_${last_file_num}.jpg
    next_overlay
    rm -rf .tmp
    if $print ; then print ${save_dir}/${prefix}_${last_file_num}.jpg ; fi
}

function countdown() {
   beep -f 1000 -r 6 -d 600 -n -f 1000 -r 5 -d 0 
}

function print() {
    lp -o media=4x6.bl,prophoto2 -o scaling=100 -o brightness=103 $1
}

function next_overlay() {
   if [ $current_overlay -lt 2 ]; then
       next_overlay=$((current_overlay+1))
   else
       next_overlay=0
   fi
   echo $next_overlay > ./overlays/current
}

# if arguments is equal to 4, process images
[ $# = 4 ] &&  process "$1" "$2" "$3" "$4"

# if arg is print
[ "$1" = print ] && print $2

# gphoto hook script
if [[ $ACTION ]]; then

    if [ $ACTION = start ]; then
       if $beeper ; then countdown ; fi
    fi
    
    if [ $ACTION = stop ]; then
        process ${overlays[$current_overlay]} capt0000.jpg capt0001.jpg capt0002.jpg
       rm ./count
    fi

    if [ $ACTION = download ]; then
        # prevents countdown after the last shot.
        COUNT=$(cat ./count) 
        if [ $COUNT -lt 2 ]; then
            COUNT=$[$COUNT + 1]
            if $beeper ; then countdown ; fi
            echo $COUNT > ./count
        fi
    fi

fi

. . . on the raspberri