Make sure you have installed Image Magic

sudo apt-get install imagemagick

And then, from the command line:

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