ImageMagick is a powerful image manipulator, best used in a terminal in Linux.
For complete instructions see: http://www.howtogeek.com/109369/how-to-quickly-resize-convert-modify-images-from-the-linux-terminal/
In brief, for width (preserving the aspect ratio) (change the 200 for your desired width in pixels):
convert example.png -resize 200 example.png
for height (still preserving the aspect ratio):
convert example.png -resize x100 example.png
It is also possible (and easy) to process several images at the same time. For example, the following command would take all PNG files in the current directory, give the all a width of 600px and save a new copy of each with “-600px” added to the beginning of each file name. The directory will now contain the old and the new image files.
for file in *.png; do convert $file -resize 600 600px-$file; done
For complete instructions see: http://www.howtogeek.com/109369/how-to-quickly-resize-convert-modify-images-from-the-linux-terminal/ (repeated for emphasis!)
man convert in your terminal or visit http://www.imagemagick.org/Usage/ for a multitude of things that can be done with imagemagick.