Force HTTPS on Thumbnail > Lightbox (dreaded IE warning)

I am creating a shop and it has to work completely via HTTPS (SSL).

We have an SSL certificate.
Installed the Wordpress HTTPS plugin (which I find to be very useful on many projects).
Also changed the .htaccess file to force all files to use https

RewriteCond %{HTTPS} !=on [NC]
RewriteRule ^.*$ https://%{SERVER_NAME}%{REQUEST_URI} [R,L]

But, no matter what I do to force HTTPS on EVERYTHING, the one part that simply will not revert to a https URL is the thumbnail > lightbox link in MarketPress.

In IE we get the dreaded warning about displaying only secure items when clicking the thumbnail to view larger image in lightbox.

Do you know of a quick fix for this please so I can get it working?