[Image] How to resize, convert & modify images from the Linux

Installation
$ sudo apt-get install imagemagick

Converting Between Formats
$ convert howtogeek.png howtogeek.jpg

You can also specify a compression level for JPEG images:
$ convert howtogeek.png -quality 95 howtogeek.jpg

Resizing Images
$ convert example.png -resize 200×100 example.png
  - to force the image to become a specific size – even if it messes up the aspect ratio
$ convert example.png -resize 200×100! example.png

$ convert example.png -resize 200 example.png
$ convert example.png -resize x100 example.png

Rotating an Image
convert howtogeek.jpg -rotate 90 howtogeek-rotated.jpg

Applying Effects
ImageMagick can apply a variety of effects to an image.
  - For example,   the following command applies the “charcoal” effect to an image:
$ convert howtogeek.jpg -charcoal 2 howtogeek-charcoal.jpg
  - the “Implode” effect with a strength of 1:
# convert howtogeek.jpg -implode 1 howtogeek-imploded.jpg

Batch Processing
for file in *.png; do convert $file -rotate 90 rotated-$file; done



Reference:
http://www.howtogeek.com/109369/how-to-quickly-resize-convert-modify-images-from-the-linux-terminal/