Suppose you want resize every your photos to put it on web. As you know, you can’t upload your photos on a web page with the highest resolution because a web gallery became inaccessible and too slow to load. Suppose that you have a folder containing every photos of you holidays and you want resize it at 640 of width maintaining original aspect ratio. To make a resize there are many tools but I suggest a command line tool called “mogrify”. This tool will be installed with ImageMagick Libraries, so you have to install ImageMagick on you Linux.
sudo apt-get install imagemagick
Now you can go to your home folder which you have saved your photos and do :
mogrify -resize 640×480! *.jpg