Open lightbox in cross domain parent from inside iframe

I’m really struggling to get my head around how to achieve this:

I have an iframe whose domain is different to that of the parent it’s being called on.
I click on a link inside the iframe and need to open a lightbox in the parent window with an image url that’s passed from the onclick link in that iframe

I have seen some people talking about techniques such as:

…but this is way over my head, can any of the experts on here help me with this?