I think it is not a good idea, because it lacks general usage.
better is a general way of including any URL inside the document;
the plane was a boeing 747
and the URL would be include inside the document
just as in <img src="URL">
the alt tag could be defined too; and it even would be a reason
to forget about the <img part alltogether, because the normal
rules would apply and the browser would see that is is an image, but
that is a point that is a bit besides the track.
The real point is that you can use the <src anywhere in your document; not
just in lists.
A browser could always include them, or by means of a switch a la
'delayed image loading' could just make it clear that there is something
that could be include; but you didn't yet.
great for copyright messages and lots of other stuff.
could be cached the same way images are.
I didn't follow the specs of html+, so it may in fact already be possible.
Pei Wei's example of the folded list would easely be created with the <src