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.

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:
http://support.apple.com/kb/ht2506

On this page, under the "Viewing and working with images" section:
http://support.apple.com/kb/ht2506#images

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.

Resizing and Cropping on Windows

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:

http://support.microsoft.com/kb/169938

If you have MS Office 2003 or newer, Photo Editor has been replaced with Picture Editor. The link below provides instruction for this tool:

?http://office.microsoft.com/en-us/help/CH001000117.aspx

Finally, MS Paint comes with all versions of Windows. The most recent version is documented here:

?http://windows.microsoft.com/en-US/windows7/Using-Paint