Open folder with the images. Copy them to a new folder, since imagemagick writes over them.

click tools/open terminal

type in mogrify -resize 320x240 *.jpg

It adjusts the height to maintain the aspect ratio

very fast, but they still have large file sizes

use this to also reduce file sizes

mogrify -resize 500x375 -quality 60 +profile *.jpg

mogrify -format jpeg -quality 60 *.png