Thumbnail and Preview quality depends on a couple factors:

1. The width of the display screen

2. The width of the original image


For thumbnails in the library, assuming the width of the image is greater, then the quality will be as shown listed below:

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

Preview images:

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