Batch Resize ImagesImageMagick is just the one for it. Here's how you can get it working as well.
Step1: install ImageMagick
$ sudo apt-get install imagemagick
Step2: go to the images folder and type
$ mogrify -sample 1280 x 800 *.JPG
Here 1280 x 800 is the final size you want and *.JPG is the file set.
To get more info on command options, visit http://linuxuser32.wordpress.com/2007/06/16/batch-image-convert-scale-thumbnail-jpegs-pdf/