til/command-line-utils/manipulating-images.md

You can use convert to crop or resize your images. Let's say you have an image with size of 800x600 and you want to crop 100 px from left, 100 px from right and then resize it to 200x200. Here are the commands you would run:

convert original.jpg -crop 700x600+100+0 cropped.jpg # Crop from left. 100+0 is offset from top left corner.
convert cropped.jpg -crop 600x600+0+0 cropped.jpg 
convert cropped.jpg -resize 200x200 final.jpg