Yes, you can also transclude image tiddlers into an $image widget — and that’s a major advantage of this approach. If I decide to change my data structure in the future, all I have to do is change the <$image source= value in my template.
But like Scott, I’d like to keep my tiddler count (and my wiki file size) as low as possible; I’d prefer not to make a separate tiddler just to hold a single field value, since that tiddler will come with its own field set full of metadata I don’t actually need. And like Springer, I endeavor to keep all the information relevant to a single tiddler in that tiddler, so the context is always obvious. I think Charlie put it very well here: