During the migration of content from ldsXML to (x)HTML5 we sometimes had both a mediaID and a webURL on an image tag, when this happened we have chosen to use the mediaID as the value for the @src and we stored the webURL in this attribute. The reason for doing this it keep this value close at hand for a season just in case we need it during the migration period. It is intended to be deprecated from the content and the schema as soon as it is prudent.