In windows we need to use double slash...
httpStatic: 'C:\\Users\\user\\.node-red\\public'
In the markdown node use:
![Image Text](/test.png)
or
<img src="/test.png" alt="Test">
In this case, the real path (on disk) for image is C:\Users\\user\.node-red\public\test.png
.
This works when accessing the dashboard locally (127.0.0.1)