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