Use the 'convert' tool (included in ImageMagick) to for example generate a resized version of all JPEG images in a directory:
for img in `ls *.JPG`; do convert -resize 33% $img resized-$img; done
On Windows you cannot run the line above from the command prompt.
Instead, create a shell script called convert.sh with the line above included.
Then run sh convert.sh from the command prompt.
You have to install UnxUtils first to be able to run shell scripts on Windows.