Reduce image size on Linux

First of all, install ImageMagick.

Resize the image:

magick original.jpg -resize 30%  destination.jpg

Change the image compression level:

magick original.jpg -quality 80  destination.jpg

For more information, run:

man magick

Reduce many images in one command

for file in *.jpg; do
    magick "$file" -quality 50 "reduced_$file"
done