copyright information for image repositories (schema.org, json-ld)

This is not exactly a technical question, it's about understanding the elements of CreativeWork and ImageObject in schema.org.

When creating structured data for an image, there are some attributes I can use to convey the copyright information, notably:

• creator
• copyrightHolder
• license

This looks quite straightforward for me (even considering the legal differences of "copyright" in the Anglo-American and European worlds).

What I'm looking for is how to include the copyright information from pictures bought on image repositories like iStock. The information provided looks like: istock.com ©acreator ID-000000000.

How is this information meant to be used with schema.org?

As copyrightHolder is meant to be a Person or Organization, using the string 'istock.com ©someone' as the person's or organisation's name doesn't seem to be the right thing to do.

So I came up with this JSON-LD code:

[
  {
    "@context": "https://schema.org",
    "@type": "ImageObject",
    "contentUrl": "https://path/to/my/image",
    "copyrightHolder": {
      "name": "istock.com"
    },
    "creator": {
      "name": "someone"
    },
    "copyrightNotice": "istock.com ©acreator ID-000000000",
    "license": "https://www.istockphoto.com/en/legal/license-agreement"
  }
]

I'm still not sure where to put the ID of the image, and also I'm not happy using the id on istock.com as "name" attribute for "creator".

I could also only use the copyrightNotice and not use "creator" and "copyrightHolder" but I'm not sure if I'd meet the legal requirement then.