Problem
You want to resize images from the command line.
Solution
Use image magik mogrify command.
mogrify -resize "percentage" image_na
Also you can view current sizing – like this:
% identify IMG_3864.JPG
IMG_3864.JPG JPEG 563x422 563x422+0+0 DirectClass 8-bit 32.8887kb
Example
mogrify -resize 20% IMG_3705.JPG