Page Bundles is not the same as plain Markdown syntax images.
There are 2 ways of going about it.
The shortcode way and the template way to call .Resources images.
The template way is not ideal for many images as it’s not very flexible.
The shortcode way is the most flexible and the one that is used more often and documented.
However both of these ways don’t work with markdown editors for image previews. And I suppose that this is what you mean by GitHub compatibility.
If you use plain markdown image syntax in your content Hugo will not find them if they’re not called with the above methods.
There are a couple of things you could do here but they might cost you in the long term if you have a large site.
- See my hack over here: Add a class to images
You can modify it to include a .Resources shortcode within the alt attribute of a markdown image.
Like this: 
- And then perform a replaceRE to remove everything between the parentheses from .Content so that it’s not rendered when your Hugo site is generated.
It’s a bit of a sneaky and convoluted way of going about it but it would give you what you want. Markdown image previews and access to Hugo’s .Resources and image processing.
Now that was a long post…