center popup without javascript setting

This plugin is great, however I am unable to make it work on a responsive website. I am using headway theme - although I am able to set the image width to 100% (in both the html view and the popover size)

I can get the popup to size down on the iphone, but it displays poorly (off to the left) on the desktop - OR i can get the popup to display properly on the desktop, but will not size down on the iphone.

can someone give me a hand here?