Gimp
GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring. It works on many operating systems, in many languages. GIMP is often cited as a replacement for Adobe Photoshop
Re-sizing Images
Note: For this method to work on any Linux distribution, you must install the 'gimp-plugin-registry' package and restart GIMP.
You may often find the need to resize several images at once in order to work with them on a computer without dedicated graphics, or to upload them at a reasonable speed. To resize many images quickly, open GIMP (no file necessary) and on the Menu Bar select Filters -> Batch -> David's Batch Processor.
Now add the desired files and select the 're-size' tab. Check the 'enable' box and select the desired relative size reduction, making sure to keep the same ratio.
Near the bottom of this window, press the 'start' button.