The File Browser supports a Thumbnails view for selecting clips visually. The thumbnails can be created in various sizes. To change the thumbnail size, select a different size in the "Thumbnail Size" drop list.
Thumbnails are created on demand whenever you browse a folder in Thumbnails view. By default, thumbnails are cached on disk, so they only need to be created once; to disable this feature, uncheck the "Cache thumbnails" checkbox.
FFRend stores thumbnails in the folder to which they apply, in a database file. The database files are named according to a specific convention, e.g. for 96 x 72 in 32-bit color, the database name is ckThumbs96x72x32.db.