According to RFC-2426, the LOGO/PHOTO:
"Type value: A single value. The default is binary value. It can also
be reset to uri value. The uri value can be used to specify a value
outside of this MIME entity."

Example (Also from RFC):
PHOTO;VALUE=uri:http://www.abc.com/pub/photos/jqpublic.gif

However, QtVersit uses "URL" instead or "uri":
PHOTO;VALUE=URL:http://www.abc.com/pub/photos/jqpublic.gif

Evolution/libebook - for example - follows the RFC.