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