You should be able to make a letterhead template by placing the
letterhead in the header, saving it as an ODT styles file and
specifically referencing any images in a list in the ODT_STYLES_FILE
header. See the manual 12.12.5, "Applying custom styles".

When I try it, though, Org fails to parse the odt_styles_file list.

I attach my test files - I'd be obliged if someone else could take a
look, maybe it's just my setup.

Yours,
Christian