This can be done from terminal. How to Quickly Resize, Convert & Modify Images from the Linux Terminal:
ImageMagick will try to preserve the aspect ratio if you use this
command. It will alter the image to fit within a 200×100 area, but the
image may not be exactly 200×100. If you want to force the image to
become a specific size – even if it messes up the aspect ratio – add
an exclamation point to the dimensions:
convert example.png -resize 200×100! example.png
You can also use GIMP.
GIMP is expandable and extensible. It is designed to be augmented with
plug-ins and extensions to do just about anything. The advanced
scripting interface allows everything from the simplest task to the
most complex image manipulation procedures to be easily scripted.
Now, to install Gimp, you can:
Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the commands below.
sudo add-apt-repository ppa:lucid-bleed/ppa
sudo apt-get update
sudo apt-get install gimp