How to batch-resize images using ImageMagick

open terminal and cd into the directory containing your photos and execute below command

mkdir photos-Optimized;for photos in *.JPG;do convert -verbose "$photos" -quality 85% -resize 1600x900 ./photos-Optimi

Voila! Now you have successfully batch resized/compressed images, You can see the optimized images in the photos-Optimized directory.

Taken from https://linuxsuperuser.com/batch-resize-compress-photos-linux/