If we want to resize some image from the command line, the ImageMagic is a good choice.
After you installed the ImageMagic, you can use convert
command to resize your image.
# percentage
convert rose.jpg -resize 50% rose.png
# specify size
convert rose.jpg -resize 100x100 rose@100.png
Also convert command can convert image format.
convert rose.jpg rose.png
You may want to find more options here. Or look the help of this command.