To resize images on Ubuntu/Debian via command line, you can use a package called imagemagick. To install it, open a terminal and run:
sudo apt-get install imagemagick
Now you can resize images using an utility called convert which is installed with imagemagick. Here is an example of how to resize a JPEG file to 640x480 pixels:
convert -resize 640x480 <input-file.jpg> <output-file.jpg>
The command above takes an image called input-file.jpg, resizes it to 640x480 pixels and outputs the result to output-file.jpg.
You can also use a percentage value to resize the image. To reduce the image to 50% of its original size, run:
convert -resize 50% <input-file.jpg> <output-file.jpg>