Both Mac and Windows have basic image editors which you can use to modify images, as discussed below.
The free site ?http://www.picnik.com provides easy access to the most common photo editing tools, making it extremely easy to Resize, Crop, Color and otherwise edit images.
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.
If you have Microsoft Office 97-XP, the Microsoft Photo Editor provides a functional, if not user-friendly interface. The link below details each of the functions of the tool, including cropping and resizing:
If you have MS Office 2003 or newer, Photo Editor has been replaced with Picture Editor. The link below provides instruction for this tool:
Finally, MS Paint comes with all versions of Windows. The most recent version is documented here: