How to add a class attribute to a image

Hello,

I want to add a class attribute to my img tag.

      <informalfigure role="test1">
        <mediaobject role="test2">
          <imageobject  role="test3">
            <imagedata  role="test4" 
fileref="images/articles/common_law.png"/>
          </imageobject>
          <textobject><phrase>alt</phrase></textobject>
        </mediaobject>
      </informalfigure>

but the role attribute dont generate a class attribute.

<div class="informalfigure">
    <div class="mediaobject">
        <img alt="alt" src="images/articles/common_law.png"/>
    </div>
</div>

Any idea?

Dominic