The devs have not granted access to Marketplace images via the api. It is not specific to that script; there are other summoning scripts ( example ) that have the same limitation. I suspect it is in an effort to curtail piracy in some way, but that is only speculation, and honestly I'm not sure how it even helps do that. You can work around it by dragging the desired image to the vtt, select it and press "z", right click to save the image locally, then upload to your personal art library, drag this copy of the art out, and update the default token. A pain, I know, but that's the only way I know how to get around it.