If you are using Linux and you need to resize a hundred images or so, you can certainly use Gimp, but that would be too much work. Consider ImageMagick set of graphic tools. You can install ImageMagick on Ubuntu by going to the Terminal and typing: “sudo apt-get install imagemagick.” Once the application is installed, all you need to do is go to your image directory and execute the following command:
mogrify -resize 900x600 *.jpg
This command will resize any image with a .jpg extension to a size of 900×600 pixels.
One more thing. Make a backup of your originals.