From the performance and best practices doco:
vCloud Director only fetches thumbnails if they are requested and once fetched, also caches them. Thumbnails are requested when a user navigates to various list pages or the dashboard that displays the VM image.
Thumbnails are cached by default for 180 seconds, and can be adjusted by adding keys to the global.properties file and restarting the cell. See the Configuration Limits section of the performance and best practices document - page 23.
