Help!Adding a PNG transparent image over header

I am using a child twenty eleven theme and the structure is still the same.

What i want to do is add a transparent image so that it overlaps my header (the image is my gravatar except larger) it will be placed towards the right of the header so the default Header text (which i beleive in header.php is) …

<?php bloginfo( ‘name’ ); ?>

…will still be there

I am willing to add the code on request.