How to Resize Multiple Images in a Folder in Terminal

Need imagemagick. (Already installed).
     sudo apt-get install imagemagick

1. Make a copy of the pictures you want to resize. ** DO NOT use originals **

2. Open terminal and go to folder.
     cd <directory-location>

3. Reduce images by 50% 
     mogrify -resize 50% -format jpg *


   or

     mogrify -resize 800x600 -format jpg *




Reference:
http://www.imagemagick.org/Usage/basics/#mogrify