You can expose the image through a site. You only need to change the instance url by the site home page.
Also, you need to establish the article as public.
For example:
If you have this site:
And your original image url is:
https://c.cs20.content.force.com/servlet/rtaImage?eid=ka3m0000000XXXX&feoid=00Nm0000000XXXX&refid=0EMm0000000XXXX
Your public image would be:
https://mysite-developer-edition.cs20.force.com/home/servlet/rtaImage?eid=ka3m0000000XXXX&feoid=00Nm0000000XXXX&refid=0EMm0000000XXXX
This workarround would work with any image in a Rich Text Area.