Creates many copies of images in the correct format so that the mac screensaver shows them for a longer duration.

convert_for_screensaver.sh

#!/bin/bash

# assuming convert.sh is in the folder you want the new images to be in,
# and the source images are one level up, then run with:
# ./convert_for_screensaver.sh ../*.{JPG,jpg}

# each copy shows for about 5 seconds, so 120 copies is about 6 minutes per slide
copies=120

num=1

for i in $@
do
  command="convert $i -geometry 2560x1440^ -gravity center -crop 2560x1440+0+0 $num.jpg"
  echo $command
  eval "$command"
  for j in $(seq 1 $copies)
  do
    command="ln $num.jpg $num-$j.jpg"
    echo $command
    eval "$command"
  done
  ((num++))
done