Bash Command Line
Run script in background without parent process
This &> /dev/null & disown %- makes the parent process/script not wait for it to finish.
Rename all files sequentially
count=0 ; for i in *.jpg; do count="$(($count +1))"; mv "$i" $(printf "spelkamer_%02d.jpg" $count); done
Resize JPG files
Use an exclamation mark as -resize 800x600! to make it scale to 800x600 pixels and not use its original ratio.