There is even precedent for doing this seamlessly: the Apple Mail client will not render media from unknown senders without user confirmation. iMessage should have the exact same behavior for the same reasons. It’s frustrating to watch greedy project managers re-learning the exact same lessons that a previous generation already learned the hard way, especially when they all work in the same building.
This is for a different reason (all useful e-mail clients do the same thing!). If Mail (or thunderbird or whatever) loaded the media, then the sender could know you opened the e-mail (by sending each recipient a unique image), leaking information.
Does the Apple Mail client do this for images included with the message (instead of referencing by URL)? Like another comment mentions, this is done for other reasons, and many clients render embedded images by default.
Mail does this for privacy, not security. Emails can load remote content and this can be used to track you. This is not generally true of images sent in iMessage because they get sent directly.
