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You can expose the image through a site. You only need to change the instance url by the site home page.

Also, you need establish the article as public.

With your image url:

https://c.cs20.content.force.com/servlet/rtaImage?eid=ka3m0000000XXXX&feoid=00Nm0000000XXXX&refid=0EMm0000000XXXX

And a site home page i.e.

https://mysite-developer-edition.cs20.force.com/home

Your public image would be:

https://mysite-developer-edition.cs20.force.com/home/servlet/rtaImage?eid=ka3m0000000XXXX&feoid=00Nm0000000XXXX&refid=0EMm0000000XXXX