Question! How to resize multiple images at once.
Answer! You will first need a tool called ImageMagick, so bring up the console and type in the following command to install it :
apt-get install imagemagick
Then type the following command in the console :
for i in $( ls *.jpg); do convert -resize 50% $i re_$i; done
Note! Replace "jpg" by the extension your images have (it's case sensitive), and "50" by the percentile you want your images to be resized to. The command will resize all images with the mentioned extension in the current directory it is run from.