use "convert" part of ffmpeg package

1.jpg size 3456x4608

  convert -resize 25% 1.jpg 2.jpg

This will reduce the size to 25%, not by 25%.

------------------------------- use "mogrify" part of ImageMagick

  mogrify -resize 25% *.jpg      # keep image aspect ratio
  mogrify -resize 320x240 *.jpg  # keep image aspect ratio
  mogrify -resize 60% *.jpg      # keep image aspect ratio

see the !

  mogrify -resize 320x240! *.png # don't keep image aspect ratio
  mogrify -resize x240 *.png     # don't keep image aspect ratio
  mogrify -resize 320x *.png     # don't keep image aspect ratio