Many open a separate application to change the image size.
In the Gnome Desktop, Nautilus file manager offers to this task a nice and easy alternative.
Nautilus Image Converter Resize Images, Picture, Graphic Converter directly in your Nautilus file manager, Free Image File Converter.
Scaling process is simple:
The easiest way how to install Nautilus Image Converter in Linux Mint, open the Terminal and copy there the following command
sudo apt-get install nautilus-image-converter
After installation, log out and log in again to activate Nautilus Image Converter.
Tip: NEVER hustle the original images, but work only with COPYs. For learning, Create on your desktop a TEST folder, and copy there some pictures 5 to 10 pcs. where you can test this Image Converter, without fear that you destroy valuable pictures.
A word of warning!
This kind of powerful Image Converter tool is hardly to found in the GUI framework, so you should be careful, not to lose valuable pictures.