There are several ways to do it, depending on what computer system/software you have. Any image editing software or even basic drawing program will allow you to do this. Your camera probably came with a program to do it.

There are also a variety of easy freeware programs that can do this. If you want a powerful, free photo editor, try the GIMP. It does this along with thousands of other things.

If you use windows XP, there is a free Powertoy from Microsoft called Image Resizer. It allows you to right click on an image and resize it to one of several common sizes. You can download it at the link below.
XP Powertoys

-----
Chris Harrison
San Antonio, Texas