The dashboard widget embeds images over HTTP, even if the page is loaded with HTTPS. This creates a mixed-content warning in many browsers.
I think you should use
is_ssl() when generating the markup to insure that the protocol matches the current page. It'd be even better to host the image locally and use
Until then, an easy fix for other users is to add this statement to a functionality plugin or your theme's functions.php:
add_filter( 'xpf-dashboard-widget', '__return_false' );