just my 2cents... a better approach would be to disable linked picture display by default, for everyone and on all kinds of networks... i think this is the standard behaviour of most email clients like outlook and gmail and outlook.com, etc... as mentioned above it's to prevent needless bandwidth use and spam/ads tracking.
we have the options to enable linked picture display based on network, which is good (although it's buried deep into the setting), what we need is an option to display only linked pictures with known contacts.
but as i said, it's better to disable by default. better save than sorry.
(if you know how many friends have their accounts hacked?? i wouldn't even display images sent from known contacts by default...)