It is sometimes possible to decrease the size of the thumbnail images file using the "Pack Thumbnail Images File" option in the Tools menu. When the image file is packed, a new image file is created containing only the images used by the file. Thus, images associated with deleted files and duplicate images for link thumbnails that have been refreshed are not included. Please note that the packing process may take a while on slow machines or with very large files.
This option is normally not needed.
Image Compression Quality
Another way to decrease the size of the image file is to decrease the "Thumbnail Image Compression Quality" on the "Advanced" tab in Preferences. When image files are saved with a lower compression quality, they occupy less space. The lower compression quality applies only to images generated or refreshed after the setting is changed. To refresh all thumbnails in a file (so they are generated with the lower setting) delete the .nvt (thumbnail) file corresponding to the .nvf (favorites) file, click "Select All" from the Edit menu, and click the
button on the Toolbar to refresh all links. Following the above procedure before deleting the .nvt file will not result in all links being refreshed with the lower compression quality because the refresh feature does not generate images for links that already have images.