This article explains how to resize images on Linux and OSX. The link below is a great article for resizing many images/photos in one go. I’d argue it’s simpler on a Linux system but i digress.
http://osxmactips.blogspot.com.au/2009/04/resize-multiple-pictures-at-once.html
On a Linux system you can use the “convert” command which is part of the “ImageMagick” package.
convert -resize 1024X768 source.png dest.jpg
You can see you can convert the image type too. You can do all kinds of things with the “convert” program. Find out more here “http://www.imagemagick.org/”.