If the avatar will be made available via HTTP instead of a pubsub data node, the publisher MUST either verify that the avatar exists at the HTTP URL or publish it via standard HTTP methods (such methods are out of scope for this specification; refer to RFC 2616).
Whenever the publisher wishes to change its current avatar, it MUST update the metadata node.
+Whenever the publisher wishes to change its current avatar, it MUST update the metadata node. As with the data node, the publisher MUST ensure that the value of the pubsub ItemID is a SHA-1 hash of the data for the "image/png" content-type (the match between the ItemID of the data node and metadata node is used by the subscriber to determine if a locally cached copy can be displayed).
The following example shows metadata specifying avatar data that is available in only one format ("image/png") and accessible only at the data node.