Be careful, images are overwritten.
Resize an image to 75% of its original size:
$ mogrify -resize 75% image.jpg
Resize all JPG files in the folder 75% of their original sizes:
$ mogrify -resize 75% *.jpg
Resize all images in the folder to 640×480 pixels:
$ mogrify -resize 640x480' *
Resize all images to width of 640 pixel mantaining aspect ratio:
$ mogrify -resize width='640' *
Resize all images to height of 480 mantaining aspect ratio:
$ mogrify -resize width='x480' *