If you use Windows XP there is a free add-on from Microsoft that will make this simple. It's called "Image Resizer Powertoy" .Go HERE
,download it and install it. You can then right-click on any image and choose "Resize Image" . Then you get a window asking you to choose Small, Medium, Large, or Custom sizes. The original image remains untouched and a new one tagged with the size you chose is saved as well (example: "image1.jpg" resized to medium would be named "image1.jpg (medium)")
Hope that helps.