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

[TallTed]

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?

[BigBlueHat]

This one is editorial. Really just need clearer language--probably removing "image data" which MUST be expressed as a data: URL.