The file (xl/drawings/_rels/drawing1.xml.rels) contains the reference
to the PNG image file:
<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships"><Relationship
Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/image"
Target="../media/image1.png"/></Relationships>
It's all XML so would be possible to construct something with existing
Octave tooling.
There is an existing C++ XLSX library (OpenXLSX) so possibly an
oct-file approach may work if this is more cumbersome natively in
Octave.
If I'm following, that lets us embed an image file, but not a 'plot' in the Excel sense that can be later manipulated.