that would require changing specs of kinds 0, 1, 20 etc. "images should be only loaded from HTTP servers that return Content-Length HTTP header or specify Content-Length in imeta metadata"

This does not belong in specs because it's totally dependent on the end user. It belongs in a client. If you can't determine content length the client should offer you the option to not download it. There is no one-size-fits-all possible here. On my home connection i wouldn't even be able to tell the difference between 50MB or 50KB. If you are somewhere with near zero bandwidth you should reconsider your priorities and maybe download zero external links and settle for notes only.