This is partially supported in Markdown. You can define a label at the bottom of the document, and use that label, e.g.:
![ImageUnavailable|100x100][oops]
[oops]: upload://oqu98iBGGNSyDEolbFUxA5cJyiv.png
(Note: the label goes in square brackets.)
It doesn’t replace the whole thing for you, just the URL, but maybe that’s good for starters?