I've floated the idea of allowing various video formats in <source> for the <picture> element. Would allow for the right codec to be selected, and would easily allow for a <img src="foo.gif"> as the final fallback. Might be worth pushing on more.
Don't know if you need a spec change since this is technically now supported in Safari TP. (See the example) Really it's up to the browser to decide if it wants to use the media type or not.