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 by.