Hi I have done this in deckjs, look here
https://github.com/asciidoctor/asciidoctor-backends#examplesBasically you should do something like:
[canvas-image="./images/cosplay.jpg"]
== Slide Two
[role="canvas-caption", position="center-up"]
Hello World - Good Bye Cruel World
In this case Slide Two title will not be shown (you can also use the approach sugegsted by Jason), and moreover a message above image will be rendered as well with the title Hello World - Good Bye ....
Of course you can also create an image with a ratio 16:9 and embed it.