You have to install this acording to your system: https://imagemagick.org/script/download.php

convert myfigure.png -resize 200x100 myfigure.jpg

Or

convert -resize 50% myfigure.png myfigure.jpg

To resize multiple files, you can try the following command (as suggested by @test30)

find . -maxdepth 1 -iname "*.jpg" | xargs -L1 -I{} convert -resize 30% "{}" _resized/"{}"

Source: https://askubuntu.com/questions/271776/how-to-resize-an-image-through-the-terminal

How to change images from one format to another

You have to install this acording to your system: https://imagemagick.org/script/download.php

mogrify -format  tif *.PNG

With this command you can change all .PNG's in a folder from .png to tif. Be cautious .PNG =/= .png Linux/Unix systems are key sensitive.