Some time I must upload a lot of image to this site, but original size is to big and will take very long time to upload. So I must resize it first to at least 1024x768, but resizing a lot of image using manual or mouse click under GWENVIEW batch conversion tool is take a long time too...so I remember that imagemagic can do this using commandline, so I try it and create a script to make it easier. Below is my script:
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#!/bin/bash

for ip in $(ls *.jpg)
do
  echo ------------------------------------------------
  echo "Converting $ip"
  convert -scale $1  "$ip" "./small/$ip"
done
 ------------------------------

that's it, I save it as conv_img and make it as executable.

you can run it under directory with a lot of jpg files with command:

conv_img 20%

then it will create 20% scaled of every image and put it under "small" directory in curent directory.