Add height to your data-role div header and remove the BR.
<div data-role="page" id="page1" data-theme="a"> <div data-role="header" style="background-color:#414141; background-image:none;height:80px"> <a href="index.html"><img src="http://static.jquery.com/files/rocker/images/logo_jquery_215x53.gif" width="100%" id="headerImage"/></a> </div> </div>
When you add a link around the image, Jquery mobile adds several classes (ui-btn-left ui-btn ui-btn-corner-all ui-shadow ui-btn-up-a) to this anchor tag.
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