Does it make a difference at all whether I serve the images in an email campaign from an encrypted domain? Will this make emails less likely to end up in a client's spam folder?
It doesn't make a difference. Almost everyone uses http:// without any spam hit, so there is nowhere for https:// to improve on that, even though spam filters are an accumulative score.
If there was something in either your content or reputation flagging you as spam, having https:// linked images wouldn't award you any 'bonus points' to save the day.