Imagemagick

Imagemagick is a set of tools for manipulating images from the command line.

Installation

sudo apt install imagemagick

Convert

magick cat.jpg cat.png
magick cat.jpg -resize 50% cat.png # With resizing

Resize

magick cat.gif -resize 64x64  resized-cat.gif # Preserves aspect ratio
magick cat.gif -risize 64x64\! resized-cat.gif # Ignore aspect ratio