Speed tradeoff for when to show an image file as a thumbnail. If set to "always" then always thumbnail, even if the folder is on a remote server. If set to "local-only" then only show thumbnails for local file systems. If set to "never" then never bother to thumbnail images, just use a generic icon.
Images over this size (in bytes) won't be thumbnailed. The purpose of this setting is to avoid thumbnailing large images that may take a long time to load or use lots of memory.