If I view emails with Touchdown and images are imbedded in the email or if I open an attachment to view an image the images show up in the gallery..... So they are cached to a location by Touchdown but I save them to a specific folder if i want to save them myself.
Yes, pictures previewed in Gmail will (periodically) be cached, but not in a format that you can go and "get" or see the file on the file system of the card.