ImageMagick’s mogrify is a great command line tool to resize images.
You can resize all your JPG images in a folder to a dimension of 800×600 with this command:
mogrify -resize 800×600! *.jpg
You can resize all your JPG images in a folder to a width of 600 and  maintain aspect ratio with this command.
mogrify -resize x600 *.jpg
References
http://www.imagemagick.org/script/mogrify.php
http://www.imagemagick.org/script/command-line-options.php#resize
http://www.novell.com/coolsolutions/tip/16524.html

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