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