ImageMagick is really a convenient tool for batch image process.
Install from here: http://www.imagemagick.org/script/index.php
For my installation I simply choose what they recommended: x86 dynamic version.
Now, able to batch resize use command like( this will resize all the jpg image under the current folder (overwrite the original one) with a rescale ratio %25 :
mogify -resize %25 *.jpg
Here’s a link with more information:
Use help to get all the command line functions provided.
$ mogrify -help
Usage: mogrify [options …] file [ [options …] file …]