Could do it like HTML5 video, and use the figure tag.

    <figure>
        <img src=blah.png dpi=72/>
        <img src=blah_hq.jpg dpi=300/>
        <caption>The fabled Blah stalks it's prey.</caption>
        <copyright year=1997>John Doe</copyright>
        <license href="http://cc.org/sharealike">CC Sharealike</license>
    </figure>

And then your browser could choose the DPI most like your screen.