Textobject | alt tag

I can not get the textobject inside the mediaobject to appear as an
alt attribute in the html file after converting from sgml. Look at this
sgml code:
-----
<!DOCTYPE Article PUBLIC "-//OASIS//DTD DocBook V4.2//EN">
<article lang="no">
  <articleinfo>

    <mediaobject>
      <imageobject>
	<imagedata  fileref="./logo.png" align="center">
      </imageobject>
      <textobject>
	<phrase>Logo</phrase>
      </textobject>
    </mediaobject>
-----

I'm using dsssl stylesheets 1.78. What code could I put into my
customized dsl file to get this to work?

Lars