Thumbnail and Preview quality depends on a couple factors - 


1. The Width of the Display Screen, and,

2. The Width of the Original Image.



Thumbnails:

For thumbnails within the library, assuming the width of the image is greater, then the quality will be as shown here-


Display Window WidthThumbnail Size
<= 300 Pixels240 Pixels
300 - 400 Pixels320 Pixels
400 - 625 Pixels500 Pixels
>= 625 Pixels800 Pixels




Preview images:


For previews within the library, the image will be at most 2000 Pixels.