ImageMagick is a really cool product. Not only is it open source but its very powerful tool. My goal was to figure out how to batch resize images while keeping one side 640 pixels wide. I ended up using ImageMagick’s mogrify by issuing the command below at my CentOS shell prompt.
mogrify -resize x640 *.jpg
mogrify -resize 640 *.jpg
Depending on which side you want to be 640 pixels. If you would like for image magic to make the largest side 640 and constrain the proportions use the following command.