So there’s nothing wrong with the image per se; it’s the way the slot utterly ignores resource size on assign. This behaviour would be fine when working with a UI material (since there’s no innate size, we must rely on default size). Quite a few things rely on image size, though 
IMHO, this is a serious(ish), regressed bug unless UE5 has a magical hidden switch somewhere.
I already mentioned my image is 289x286.
I was merely thinking that importing messes things up somehow but that’s not the case.