Both Mac and Windows have basic image editors which you can use to modify images, as discussed below.
We have also found that the free site ?http://www.picnik.com provides easy access to the most common photo editing tools.
Resizing and Cropping on a Mac
Apple's Preview program (on your computer at Macintosh HD -> Applications -> Preview) is a simple, but very capable program for not only viewing files, but for performing some basic graphic editing, too. A good help link from Apple is at:
On this page, under the "Viewing and working with images" section:
you'll find instructions for resizing and cropping a graphic file entitled "Resize an image" and "Crop an image" under the "If you want to - Do this in Preview" table.