The command mogrify overwrites the original files with the resized images:

  sudo apt-get install imagemagick
  
  mogrify -resize 50% *png      # keep image aspect ratio
  mogrify -resize 320x240 *png  # keep image aspect ratio
  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