Make sure you take the picture at the highest megapixel that your camera supports, then use the resize/resample function of your graphics viewer. If you don't have a good one, here is a free program that I like.
http://www.irfanview.com/
When you install it, it will ask you about associating files to the program. I would recommend you say "no" to that until you are sure you like it. You can still load the files manually by going to "file", "open", they just won't open in this program when you double-click them.