Simply go to https://random-miku.owo39.me/ and it will redirect to https://random-miku.owo39.me/https://www.pixiv.net/en/artworks/<randomid> which will serve the raw image directly, and the url as such allows you to delete the left half to get to the source.
Your address and last requested URL are remembered so that when you click reload in your browser, you're redirected to a different one. You can add ?noredirect query param to disable this.
You can add the size query parameter with one of original, regular, small, or thumb_mini (or whatever pixiv api has) to get a different size. The default is original.
For programatic purposes you can GET /api which responds just a random ID in plain text.
The original version served already-downloaded images from a MongoDB database, containing ~90,000 of the top 初音ミク illustrations on pixiv, but the system went down so now it only has the list of ids from said database and it proxies the images from pixiv. (might revert later when I get back and resurrect said system...)
here's an embedded example.
unfortunately there's no way to get source like this (browsers don't expose redirected urls of embedded resources).