you can do it in many programs. If you don't have any photo programs just use "Paint" in windows.
Open up you file and go to Image, then to "stretch/skew" and just put in a percentage of the size you want the pic into the horinzontal and vertical fields.(obviously keep them the same number or your pic will get all messed up looking)
Then do a "Save As" and save it with a different name incase you make it too small or something you can try again with your original pic.