Random

#!/bin/bash
FILES="$@"
for i in $FILES
do
echo "Prcoessing image $i ..."
/usr/bin/convert -thumbnail 200 $i thumb.$i
done

files=(/home/hope13/pictures/*)
printf "%s\n" "${files[RANDOM % ${#files[@]}]}"

files=$( find /home/hope13/pictures/ -type f )

shopt -s globstar
files=(/home/hope13/pictures/**/*.jpg)

find ROOT_DIR -type d | while read dir; do find "$dir" -maxdepth 1 -name '*.jpg' | sort -R | head -1 ; done | while read file; do echo convert -thumbnail 200 "$file" "$(dirname "$file")"/thumb.jpg; done