This is probably easiest way to resizing image on Linux Box. Why do you need to resize.
While taking pictures using camera, many people keep settings to max MP (Megapixel), so that same snap can be used to print bigger size. This images are higher in size & if you are willing to carry in your phone, it may eatup storage.
This is when tools like mogrify comes handy, you can also use convert / gimp for this very purpose.
Resize Image width to 640
mogrify -resize 640 *.jpg
Resize Image to 125×125
mogrify -resize 125x125 *.jpg
You can find many other options using man mogrify