Why does Preview lower image-quality
The Preview does not lower the image quality, but will fit it in the given parameters.

If you use Preview to open a original image: Example:

5.5 MB file size with Dimensions of 3648 × 2736 pixels

Now open the Preview Tools - Adjust Size to see the current settings.

The default resolution is set to 72 which is standard for web applications.

In that window you can set it to the original size/resolution.



Now if you are using a copy/paste to move the image to Preview, the resulting quality will depend on the copy/paste settings. That can be found in the application it self.