Imagemagic resize photo

$ for i in $( ls *.jpg); do convert -resize 50% $i re_$i; done
The command above will resize all images to half of its original size. New resized images will be saved with a prefix "re_". It is also possible to resize all images and at the same time convert them to gif format:
$ for i in $( ls *.jpg); do convert -resize 50% $i $i.gif; done