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.
You can use the dropdown menu in the 'output' tab to convert to whichever image formats you'd like.
Near the bottom of this window, press the 'start' button.