ImageMagick resize with preserve aspect ratio.

ImageMagick is used to change images.
In this case, I will show how to resize images and preserve aspect ratio:
...with height: 600 pixels

convert  -resize x600 *.png

...with width: 600 pixels

convert  -resize 600x *.png

If you know more about ImageMagick send me one email.

Simple convert images using ImageMagick - convert features

Can be done by using ImageMagick.This is a software suite to create, edit, compose, or convert bitmap images. It can read and write images in a variety of formats over 100. Using ImageMagick let you resize, flip, mirror, rotate, distort, shear and transform images, adjust image colors, apply various special effects, or draw text, lines, polygons, ellipses and Bézier curves.
Install the software under Fedora:

# yum install  ImageMagick 

Convert an image from one format to another. In this case jpeg.

$ convert *.png *.jpg

$ mogrify -format png *.jpg  

Read more about this software here.