One easy way to do that is to open the picture on your desktop. Some picture viewer applications automatically will resize the picture while you are viewing it. If yours does, then great. If not this will still help you out.
When you have the picture opened on your desktop, hit the button on your keyboard that says "Print Screen" or "Prnt Scrn". Then open up windows 'paint' application(START/PROGRAMS/APPLICATIONS/PAINT in Windows 2000) and hit Control-V OR right click and pick paste.
This will put a snapshot of your desktop, of when you hit print screen, onto the Paint application. You can then cut/crop/adjust the snapshot to only show the picture you had open without the rest of your desktop.