I don't think we should implement that feature. It has various problems:

• Introduces syntax that doesn't work by default (e.g. it is not an optimisation, but a requirement for functional rendering as by default in a browser filepath: would point no-where). This hinders development mode as well.
• It violates the boundary between the isolated ResourceLoader system and MediaWiki user-generated content. Thus requiring that we track updates to that file within MediaWiki and somehow dispatch updates to purge and regenerate the gadget module that uses this. Such logic doesn't presently exist and can't be prioritised at the moment. Ideas are welcome however.
• It may also detriment page load performance. Recent research indicates that the habit of encouraging embedded images is actually negative to page load performance. In 2010 this was useful to reduce the total number of HTTP requests. But in 2015 we should probably focus our optimisation on SPDY/HTTP2 which the majority of users support. In that protocol, browsers have a persistent session to the web server. As such, requesting additional files doesn't have any notable overhead. It has the benefit of allowing the page to have its first render without all images (e.g. waiting 1 second to start reading the page, instead of 5 or 6 seconds).

Using images will still work, but you'll have to use the https://upload.wikimedia.org/... url instead.