ImageMagick is a free utility that is installed by default in many Linux distributions. It provides a command line interface to perform a multitude of image manipulation operations. In this post I'll show how to resize an image.
There are two different commands that can resize an image; mogrify and convert. mogrify will make changes to the existing image. convert will make the changes and write them to a new image.
Say you have an image called test.png that you want to shrink to half size. You can shrink it from the command line with the mogrify command:
mogrify -scale 50% test.png
Similarly, you can double it's size with:
mogrify -scale 200% test.png
Now if you want to keep your original image intact, you can use the convert command instead.
convert test.png -scale 50% test.small.png
or
convert test.png -scale 200% test.big.png
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There is a utility which allows you to do this resizing by simply dragging and dropping your images on an icon:
http://tabish.freeshell.org/picture-resize.html