If you are serving your content from a different domain than the page you have added Greta to, you need to enable CORS. This setting is simply a way for your server to tell the browser that it's OK to load external content.

In order to read data about your images, you need add following header:

Access-Control-Allow-Origin: *


You can test if CORS is configured on a specific url. Enter the url you want to test on in the url field and hit test