Imagemagick

This method was sooooooo much easier than using GIMP or Photoshop.

Install it and then navigate to the folder where the pictures you wish to resize are located. Type:

mkdir resized

This will create subfolder called resized in that directory. This is where your resized pictures will be located. Now type:

find . -name "*.jpg" | xargs -i convert -scale 50% {} ./resized/{}