This is a very specific/niche usecase. Using an HTML card seems like the correct approach to me here - which is designed to give you full control of markup when you need it - exactly like for things like this.
You don’t need to upload the image to an external service, however, you can upload the image to ghost, copy its image location by previewing the post, then paste that into the HTML card.
Using decorative images in post content seems like the hacky workaround that is hard to explain tbh.
Accessibility is indeed important, that’s exactly why screen readers read out file names - which is not functionality that should be disabled except for in very specific circumstances where the image no semantic meaning.
If you’re making images part of your content with no semantic meaning, then you should probably consider using CSS, JavaScript, putting them in the layout, or really anything else that doesn’t spam post content with meaningless images.
A good system does not make it as easy as possible to mis-use accessibility features. That is not a feature of a good system at all.
If the image is a portrait of a person in the context of something about that person, then empty alt text is something recommended by the WAI: WebAIM: Alternative Text (Example 1 & 2), so I would suggest that this is not a hacky workaround.