URIs are a perfectly valid "serial", and only need a few bytes (e.g. see URL shorteners). Receivers can look up images in a cache before trying to download them

URIs are a perfectly valid "serial", and only need a few bytes (e.g. see URL shorteners). Receivers can look up images in a cache before trying to download them, and caches can be pre-populated with thousands of popular images, by OS vendors, application providers, users, etc.