Great plugin, very happy with it. I'm using WPMS and domain mapping however and the widget appears to fall down when I do so. As far as I can see, all variables are generated on the back end but (for complicated and boring reasons) I'm using one domain for the backend and another for the front. This means that on my public-facing domain (cached through Akamai) the source URLs for my images (with your widget) are all running on the admin domain.
They are served and load fine, but they aren't cached. Can you patch the plugin to generate the src URL for the images based on the site URL on page load rather than fixing it on the back end?