For HTML output, readers appreciate when alt attributes are added to IMG tags to provide a text alternative to the graphic. This is particularly important for accessibility to vision-impaired readers. Also many browsers display the alt text when the mouse hovers over an image.

You can provide the alt text by including in each mediaobject element a textobject element containing a phrase element. For example:

<mediaobject  id="MousePicture">
  <imageobject  role="html">
    <imagedata  format="PNG"  fileref="mouse.png"/>
  </imageobject>
  <imageobject  role="fo">
    <imagedata  format="SVG"  fileref="mouse.svg"/>
  </imageobject>
  <textobject>
    <phrase>Picture of mouse</phrase>
  </textobject>
</mediaobject>

When processed into HTML, the output looks like:

<img  src="mouse.png" alt="Picture of mouse">

The text you provide should be plain text, since attributes like alt do not accept markup tags. Text entities and character entities are supported, however.