You cannot mark messages as 'safe' on a per-message basis, but you can add trusted individuals to your Safe Senders list so that images, links, etc. are never blocked in emails sent from them. You can find this list by going Tools->Safety Options->Safe Senders. You can also add a sender to that list when viewing an email sent from him/her.

It is also possible to disable the security feature by going Tools->Safety Options->Security and unchecking the 'Block images' box, though I recommend against doing do.

Hope this helps,
John