WordPress HTTPS Plugin 3.1
I suspect this problem needs to be fixed by the HTTPS Plugin author, but I thought I'd post this here just in case Atahualpa is doing something funny. This is largely a copy of my support request at the WP Support for HTTPS Plugin forum.
Since upgrading HTTPS to 3.1, a small incompatibility is exhibited between this plugin and the Atahualpa theme. The HTML for the header image somehow loses a single quotation marker, so no image appears. As a specific example, when the WP HTTPS plugin is enabled, the following HTML is output on the front page of faculty.missouri.edu/~umcasphysics:
<div id="imagecontainer" class="header-image-container" style="background: url('http://faculty.missouri.edu/~umcasphysics/wp-content/themes/atahualpa/images/header/banner2.gif) top center no-repeat;">