The viewer is insanely forgiving (it is from part of a Web Browser . We all known that there is a ton of cr*p code out there)). Always test on a target device.
That block must be a P or Div block (those are Block Level types (as well as H#)
An <img> ...</img> by it self is a naked set of tags. I think the Calibre Editor
debug should have flagged those.
I've add quite a few
Alternate Covers (they are JUST images of the ones on my paperbacks and are NOT flagged as 'cover')
Remember most semantics only allow ONE usage per book AND targets can not overlap It can't be a Cover AND also a TOC