Great article, I am ashamed to say I wasn’t familiar with the data attribute so in the past I have put that type of data in the id or class element.

Rather than not rendering an image at all wouldn’t it be better to load a default avatar and just replace the image with the correct once everything has loaded?
The browser would only see one request as its the same file and the user wouldn’t see a drastic change to the presentation when the avatars load.