epub: admonition icon images missing from contents.opf

When admonition icons are specified epubcheck generates validation
errors like:

ERROR: article.epub/OEBPS/index.html(4):
'OEBPS/images/icons/note.png': referenced resource exists,
but not declared in the OPF file

i.e. The admonition icon is in the EPUB file but DocBook XSL has not
been added to the content.opf manifest. Compare this with callout icons
which are processed correctly.

Environment: DocBook XSL 1.75.2; Ubuntu 8.04