[osi]

imessage won't auto-load previews until you ask it to do it the first time.

[yoz-y]

But there is no way to disable this once you have accepted it. I do not actually remember having been given the choice but it has been some time so I probably just do not remember.

Ideally one could enable previews only from contacts.

[osi]

correct - i couldn't find a way to disable if you've changed your mind.