It's adding a link back to your site for it's style sheet on all pages. This is causing pages which are using https to show as mixed mode. Is there any way to eliminate it from putting the style sheet entry into https pages or at least the user logon pages? Or having it not pull data from your site at all, like local served css?
<link rel='stylesheet' id='pdrp_styles-css' href='http://app.photodropper.com/css/main.css?ver=3.4.2' type='text/css' media='all' />