wikilinks and images aren’t parsed in yaml (wikilinks might be clicable now but as far I know you don’t get an embedded preview of them), and the cover/banner image functionality was provided by custom css or plugins/themes not by vanilla Obsidian, for example here
(but I might missed something)
On my Obsidian
cover:: ![[example_image.png|200]]
just display image, the same way as
![[example_image.png|200]]
I would say what you requesting is a new feature, not a bug, but devs or others might comment on it (there is no property type “image” that will be be rendered as I read in the release notes)