Ubuntu : Ubuntu Image Resize. On terminal

Ubuntu : Ubuntu Image Resize. On terminal
1. Install software

1. sudo apt-get install imagemagick

2. Use Command. The command mogrify overwrites the original files with the resized images:

1. mogrify -resize 50% *.jpg      # keep image aspect ratio
2. mogrify -resize 320x240 *.jpg  # keep image aspect ratio
3. mogrify -resize 320x240! *.png # don't keep image aspect ratio
4. mogrify -resize x240 *.png     # don't keep image aspect ratio
5. mogrify -resize 320x *.png     # don't keep image aspect ratio

https://askubuntu.com/questions/1164/how-to-easily-resize-images-via-command-line