Outlook gives a warning message, saying that it will download the images if you want to forward the message. I think that is unfortunate as well.
I would maybe prefer a dialog: remove references to external images/content or leave as is. The latter would show (download) or not show the images when you edit and forward, depending on whether you downloaded the images in the first place.
If that is too complicated to do, then I would expect the default behaviour to be that the images are not shown (downloaded) unless I actually chose to download them in the first place.