mogrify : ImageMagic

Install ImageMagic :

apt-get install imagemagick

mogrify - With the use of resize an image, blur, crop, despeckle, dither, draw on, flip, join, re-sample, and much more. Mogrify overwrites the original image file, whereas, convert(1) writes to a different image file

e.g Let's resize images

mogrify -resize 500x300 Test/*.png