Jim-
It is a 2-step process. First, change the canvas size to 640x 480, then save it as a different file name (otherwise you won't have your original unmodified picture). When you are saving the picture, you should be able to set/adjust the file compression so that you can make it small enough to fit within the limits.
Even though the file will look small on your screen, it should be fine when post it to the gallery (at least that's how it works for me anyway).
Good luck and I look forward to checking out your pics.
