That's a good question and one that gets asked a lot. If you are using a normal PC (not a Macintosh) and it has Windows on it, do this....

1) Click "Start" in the bottom left
2) Go to "Programs" and then "Accessories" and click on the "Paint" program.
3) Once in the paint program, under the "File" menu select "open" and choose the image you want to resize. You may have to change the directory to the location the file is stored.
4) Once the file is opened in Paint, click on the "Image" menu and select "Stretch/Skew (Ctl+W)"
5) Change the horizontal and vertical percentages to something less than 100 to shrink and over 100 to enlarge. For example, an image that is 1280 x 960 pixles scaled to 50% would make the image 640 x 480 pixels.

voila