Resizing is one of the very basic operations if you work involves playing with images. There are many tools - both GUI and command line-based - that let you do this. If you are on a Linux distribution that has Nautilus file manager, you can also add an option to resize images in the right-click menu.
This you can do using the nautilus-reduceimages tool. In this tutorial, we will quickly discuss how to install and use the tool. Please note that all examples/instructions mentioned here have been tested on Ubuntu 16.04 LTS.
Use the following commands to install the tool:
sudo add-apt-repository ppa:atareao/nautilus-extensions
sudo apt-get update
sudo apt-get install nautilus-reduceimages
After the aforementioned commands are successfully executed, run the following command to restart Nautilus:
Now, when you do a right-click on any image, you'll see an option dubbed Reduce images... :
While the sub-option 'Reduce image' does the resizing operation, it's the 'Configurate' option where you can specify the new width and height for the image (and of course, several other details).
So before every resize operation (assuming different resizing each time), you need to specify values in 'Configurate' option window, and based on this configuration, resizing is done.
This brings us to a major limitation of the tool: the resizing operation is done by adding a background canvas of the new width and height. The following resized image should give you a better idea of what we're talking about:
The other limitation I noticed is no option to preserve aspect ratio. If you are ok with these two limitations, nautilus-reduceimage is a nice little tool that's easy to install and even easier to use.