OK, problem solved. It's a work-around that should not have been necessary, but at least the reader will not know that. I have a css class called "divider" which is usually just a row of 5 centered asterixes in a slightly larger font-size, for chapter break-points, so I made a small version of the illustration and embedded it in a divider, 2 asterixes each side... it looks intentional, because it appears at a natural break-point, anyway.
It raises an interesting question, though... do established formats like MOBI undergo occasional upgrades to accommodate new demands or are they now set in stone? I know that ePub is still very much a work in progress, but we poor Kindle owners can't afford that luxury.