I read the documents and look like you need to have slimerjs http://docs.casperjs.org/en/latest/events-filters.html in order to get responseData.body from the page.resource.received event.
My use case is to download the images when the page is loaded so I don't do another round trip of: get the resource JSON, download and save any image file, reload the file to check for image dimension and if too small (icons) -> eliminate.
I am wondering if there is a better way to do this. I could in fact go do evaluate img selector but some websites use background-url css and that's tricky.