Is it `the value MUST be a URL`, or `URL or image data`?

_Originally posted by @TallTed in https://github.com/w3c-ccg/verifiable-issuers-verifiers/pull/42#discussion_r2795449638_

If `the value MUST be a URL`, then `URL or image data` isn't accurate. Should provide clearer guidance, perhaps ```A URL, which may be in the [`data:` scheme](https://datatracker.ietf.org/doc/html/rfc2397)``` or similar?
