Hi Ben,
As you’ve seen, only the “Umbraco Content” data-source has the “image” alias field. The only reason for this is that I needed it for a demo and didn’t think of adding it to other data-sources at the time. 
The best option is to open an issue on my GitHub repo, and I’ll try to add the feature soon, (or if you have C# skills, feel free to send over a pull request 
), then I’ll try to get a new release out (which may end up taking me a couple of weeks - as I’m limited on free time these days).
The more immediate option is to write a custom data-source, (again needs C# skills), that takes inspiration from “Umbraco Content by XPath” source-code., adding in the imageAlias bits from “Umbraco Content” source-code.
Cheers, Lee.