If you have source like the following in your input DocBook:
When the stylesheets generate the <item> elements in the package.opf file, they will include the attribute properties="svg" even though the svg file is not in the output epub files. This causes a problem with epubcheck (3.0b5). Otherwise, the stylesheets do exactly the right thing. They select the file specified in the imageobject element with role="html" and drop the other one, and all the links and file references are correct.