The inline attachments will be downloaded once the button is pressed. Before they are downloaded, you will note a red x in a blank box(3) on screen -- this is the placeholder for the content that is going to be downloaded. Press the Download Now button to show the content.
Besides the improved load time associated with only downloading content on demand, this method of handling the previewing of a message has the benefit of not showing potential spam content immediately when a message is clicked. This gives the User the option to delete a message before showing what could be offensive or otherwise inappropriate spam content.
Override for Contacts
In Preferences, Users have the option to override the Download Now button, always downloading images automatically for Contacts that are in the system. This is a nice option to set for people that you know already since it will always just download their signatures and other images they might insert into emails for you.