A new copy of your original image is created only if you alter it in some way within the work-window - such as resizing it, adjusting the colour settings, etc. This 're-rendering' is done for two reasons - so that the original image is not altered, and so that it is re-sampled (if stretched, to produce a cleaner looking image, and if shrunk to produce a smaller file size).

You can change the settings so that the image is not re-rendered by selecting the image, then unchecking 're-render' in the Quick Editor. The original image will then be used. Keep in mind however that you will not have the benefits described above.