You will always loose a bit of quality when doing digital zooming on graphics.
An image consist of X*Y number of pixels. So if you imagine having a 50*50 pixel image, and you zoom in on it, suddenly the details that you used 50*50 pixels to show now uses 400*400 pixels. You will loose quality, as the original images doesn't have enough information in it to allow it to maintain quality when having to use more pixels.
It is generally always best to scale down than up in my opinion, so if you know that you have to be able to zoom in by a factor of 2 (double size) you can make the image so it looks fine when zoomed in and then zoom out afterwards. But again you will loose a bit of details, but it will look better than if the image is to small and then zoomed in on, i think.