Hint: You only need one of each aspect ratio. For example, 640x480, 800x600, 1024x768, 1152x864 and 1600x1200 are all 4:3 aspect ratio. So for that set, you would only need one of 1600x1200 size. Why the largest size? You can scale down without losing quality. But you cannot scale up from, say, 640x480 to 1600x1200 without losing density and definition.