The below doesn't work on GitHub. It seems that GitHub caches and serves its own version of the image. This will work anywhere where you aren't caching images.
Here is an image from unsplash.com.
I don't know what image I am showing you though. That's because the URL
being sourced for the above image is
http://source.unsplash.com/random/1200x600. This tells unsplash to
randomly serve us a 1200x600 image. Try refreshing the page and you'll see
that it is different each time. Cool!
This is a bit of a novelty, but could be useful on occasion. The Gatsby docs use it to great effect in a tutorial so as to not get caught up with the details of serving a specific image.