I think it makes sense in your implementation, but that seems very specific to IA Writer. It doesn’t seem that portable, and at least to me, was a little confusing using the leading /.
I’ve just started working with a large corpus of markdown documents. They standardized on having an img directory to store any images. So they reference the image as img/photo.jp. It keeps the images from being mixed in with the list of documents, so it’s much easier to find something when you’re looking on your filesystem or git repo. At least for this use case, I find it works extremely well.
In any case, I would still like to see a consistent way of handling spaces in titles and urls, particularly using quotes, like "some photo.jpg" or "Photo Caption and Title"